Do we know if there exist true mathematical statements that can not be proven? Given the set of standard axioms (I'm not asking for proof of those), do we know for sure that a proof exists for all unproven theorems? For example, I believe the Goldbach Conjecture is not proven even though we "consider" it true.
Phrased another way, have we proven that if a mathematical statement is true, a proof of it exists? That, therefore, anything that is true can be proven, and anything that cannot be proven is not true? Or, is there a counterexample to that statement?
If it hasn't been proven either way, do we have a strong idea one way or the other? Is it generally thought that some theorems can have no proof, or not?
 A: This is a try to summarize the most important results of Gödel's theorems.


*

*A statement is provable within a theory if and only if it is true for any 
interpretation allowed in this theory.

*If a statement is true for some interpretation (model) and false for some other,
then it is independent of the theory and undecidable within the theory.

*But the fact, that a statement is undecidable within a theory, cannot be proven
within the theory itself. A stronger theory might prove this undecidability, or
might not.

*Any theory, that is strong enough, that the representation theorem holds for it,
is incomplete, that means, that there are true statements, not provable within it.

*Finally, a theory cannot prove its own consistency.
A: It is disturbing that in the extensive discussion on this topic, Gödel's work (his Incompleteness Theorem) has been referenced several times but there has been no mention of Alan Turing (the Halting Problem) or Emil Post (his production systems).  All three independently proved the same thing, that in any proof system there are some true statements that cannot be proven (incompleteness) or else the proof system will also prove some statements that are not true (inconsistency).  Curiously, these three developments were accomplished independently at very roughly the same time.  As Robert Heinlein posited, when the time comes that railroading is possible it will arise independently in multiple places.
A: Gödel was able to construct a statement that says "this statement is not provable."
The proof is something like this.  First create an enumeration scheme of written documents.  Then create a statement in number theory "$P(x,y,z)$", which means "if $x$ is interpreted as a computer program, and we input the value $y$, then the value $z$ is the output."  (This part was quite hard, but intuitively you can see it could be done.)
Then write a computer program that checks proofs.  Creating proofs is undecidable, and it is hard to create a program to do that.  But a program to check a proof can be created.  Let's suppose this program becomes the literal number $n$ in our enumeration scheme.  Then we can create a statement in number theory "$Q(x)$"${}={}$"$\exists y:P(n,\text{cat}(x,y),1)$".  Here $\text{cat}(x,y)$ concatenates a written statement in number theory $x$ with its proof $y$.  So $Q(x)$ says "$x$ is provable."
Now construct in number theory a formula $S(x,y)$, which means take the statement enumerated by $x$, and whenever you see the symbol $x$ in it, substitute it with the literal number represented by $y$.
Now consider the statement "$T(x)$"${}={}$"$\text{not} \ Q(S(x,x))$".  Let's suppose this enumerates as the number $m$.
Then "$T(m)$" is a statement in number theory that says "this statement is not provable."
Now suppose "$T(m)$" is provable.  Then it is true.  But if it is true, then it is not provable (because that is what the statement says).
So "$T(m)$" is clearly not provable.  Hence it is true.
I know I am missing some important technical issues.  I'll answer them as best I can when they are asked.  But that is the rough outline of the proof of Gödel's incompleteness theorem.
A: 
Given the set of standard axioms [of some mathematical theory], do we know for sure that a proof exists for all unproven theorems? For example, I believe the Goldbach Conjecture is not proven even though we "consider" it true.

In addition to all the interesting discussion about Godel's and Goodstein's Theorems, I want to suggest also another "thread" of discussion, regarding epistemology of mathematical knowledge.
During the '60s and '70s, the philosophy of science debate was concerned with the distinction between :


*

*the Context of Discovery and the Context of Justification.


Roughly speaking, the context distinction regards : how science (e.g. physics) discover a new fact or law; the second is: how science explain it (ref.Paul Hoyningen-Huene, On the Varieties of the Distinction between the Context of Discovery and the Context of Justification, 2002).
Applied to mathematics, this points to the difference between  :

the discovery of a new math idea or concept vs the proof of a theorem.

As far as I know, very few philosophers of mathematics are concerned with this kind of issue ; the only book I've read about something similar was Lakatos' Proofs and Refutations, (1976).
The connection I see is this :

when we don't have a proof of a mathematical "fact" , what are the gorund for asserting or believing it ?

Here some comments about comments in the above debate :

a) "not all true statements are theorems, that is the content of Gödel's incompleteness theorem"

They are not theorems of the formal arithmetic in question (i.e. first-order PA) but THEY ARE proved via Godel's "construction" provided by G's Theorem itself (i.e.proved in the meta-theory): isn't it ?

b) "Goldbach's conjecture is not proven, but it is almost surely true due to statistical evidence"

Are there research about "inductive" grounds for unproven mathematical facts ?
A single contradiction con destroy a theory (Russell's Paradox in front of Frege's system) but how many years (?) of absence of contradiction can support our sound belief in a theory (e.g.ZFC) ?
A: "Can't be proven" is an inappropriately vague notion for the question you want to ask.  Proven from what axioms?  In a logical system that includes Goldbach's conjecture as an axiom, the proof of Goldbach's conjecture is only one line long.  So to have the question make sense, you can't just say "proven"; you have to say "proven from such-and-so axioms".
There is a standard set of axioms for arithmetic, called the Peano axioms. We like these axioms because they are intuitive and simple, and also because they seem to be powerful enough to prove almost all of the things we'd like to prove about arithmetic.
However, it is known that there are particular true statements of arithmetic that are not provable from the Peano axioms; Goodstein's theorem is an example.
Gödel's famous incompleteness theorem states that any system of axioms that is expressive enough to prove all true statements of arithmetic must also prove some false statements of arithmetic. Conversely, any system of axioms that proves only true statements of arithmetic must fail to prove some true statements of arithmetic.  The proof is constructive; starting from the given axioms, it constructs a (highly artificial) statement of arithmetic $G$ which is true if and only if there is no proof of $G$ from the axioms. Either $G$ is false and has a proof, or it is true and it has no proof.
A: Let $\Sigma$ be Rado's Busy Beaver function. 
There exist specific positive integers $k$ and $n$ such that $"\Sigma(k) = n"$ is true-but-unprovable in ZFC. 
Also see this question and answer.
A: Gödel's incompleteness theorem is one of those widely misunderstood results.
Roughly it means in the context of arithmetic you can only have two of the following:


*

*Decidable axioms

*Consistency

*Completeness


The "truths that cannot be proven" is an abbreviation for the context of choosing decidable axioms, consistency, but a lack of completeness. This means there are sentences P for which there is no proof of P or not P.
You can throw in more axioms of arithmetic so that every sentence P has a proof of P or not P. That will give completeness, consistency, but the axioms will necessarily be undecidable because of Gödel's incompleteness theorem.
A point that is often missed in the statement "truths that cannot be proven" is that it is meaningful to speak of undecidable, complete, consistent axioms of arithmetic where every true sentence can be proven. But it comes at the cost of undecidable axioms which is why its not particularly useful.
A: Amongst the many excellent answers you have received, nobody appears to have directly answered your question.
Goldbach's conjecture can be true and provable, true but not provable using the "normal rules of arithmetic", or false. 
There are strong statistical arguments which suggest it is almost certainly true. 
Whether it is provable using the "normal laws of arithmetic" - like those used to prove Fermat's Last Theorem or the Prime Number Theorem and everything you learned in high school maths - is not known. Assuming it can't be proven is a complete dead-end. To be interested at all you have to either assume it is true and be looking for a proof, or assume it is false and be looking for a counter example.
A: 
do we know for sure that a proof exists for all unproven
  theorems?...Phrased another way, have we proven that if a mathematical
  statement is true, a proof of it exists?

Perhaps this is a semantic issue, but I don't think these two questions are identical, even though you may have intended them to be so.
If a mathematical statement has been proven as true, and the proof is correct, then yes we have proven that a proof exists. It doesn't mean that it's the only proof possible. In the book "Proofs Without Words" there's around 13(?) proofs of the Pythagorean Theorem with no words at all, just diagrams.
For statements that haven't been proven yet, there are two possibilities. Somebody can prove that the statement can't be proved by pointing out a flaw in the reasoning used to arrive at that statement. Alternatively, if you can prove that a contradictory statement to be true, you have proven the original statement false - therefore no proof can exist. 
A: Euclid's fifth postulate:
Two lines are drawn which intersect a third in such a way that the sum of the inner angles on one side is less than two right angles, then the two lines inevitably must intersect each other on that side if extended far enough. This postulate is equivalent to what is known as the parallel postulate.
Euclid's fifth postulate cannot be proven as a theorem, although this was attempted by many people. Euclid himself used only the first four postulates ("absolute geometry") for the first 28 propositions of the Elements, but was forced to invoke the parallel postulate on the 29th. In 1823, Janos Bolyai and Nicolai Lobachevsky independently realized that entirely self-consistent "non-Euclidean geometries" could be created in which the parallel postulate did not hold. (Gauss had also discovered but suppressed the existence of non-Euclidean geometries.)
