I (think) that I once heard it said that from Tarski's Theorem on the undecidability of truth, Gödel's Second Incompleteness Theorem follows. Is this right?
Tarski's theorem is on the unDEFINability of truth, but yes, Goedel's results follow (and in some sense are the same thing): provability (in some particular system of interest) is definable, therefore truth cannot match provability, therefore (supposing the system of interest is consistent, and therefore sound for the relevant sentences) there is a sentence which is true but not provable. This is the first completeness theorem. What would it mean for a predicate T to actually define truth? Just that T(s) and s are equivalent for each sentence s. Thus, Tarski's undefinability theorem tells us more specifically that, for any definable T, there is some sentence G such that T(G) and G are equivalent to each other's negation. The above is just what happens when we take T to define provability; we thus see more specifically that incompleteness arises from a sentence G equivalent to the negation of its provability, which is therefore (supposing the system of interest is consistent) true but not provable. One can then argue, in the usual way, that our proof of the first incompleteness theorem, suitably internalized in the system of interest, yields the second incompleteness theorem. [Our argument shows that the consistency of the system of interest entails the nonprovability of G. But this conclusion is just what it means for G to be true! Thus, if the system of interest proves its own consistency, it proves G, contra its own consistency.] 

