As we've seen from Mitt Romney's answers on TV recently, there is obviously something in his tax returns that he doesn't want to be made public.
Imagine this though: What if Mitt Romney were actually elected president without releasing 10 years or more of tax returns?
If there is something truly politically damaging in there (as Romney himself has practically confirmed), it is possible that a foreign government (through espionage, etc.) could learn of the damaging information, and actually blackmail the President.
The release and vetting of Mitt Romney's tax returns is actually a matter of national security.