In the 2008 election, Mitt Romney gave John McCain 23 years of his tax returns in the hopes that he would be McCain's VP candidate. McCain turned him down in favor of Sarah Palin, later explaining "We thought that Sarah Palin was the better candidate." So he was okay with releasing 23 years of his tax returns in order to get the job of Vice President, but not President? Hm. What does John McCain know that the American people don't?
I personally think that Mittens should have released his tax returns, as it might have helped him in the Election. But he really had no obligation to; it's his business. The accusation that he didn't pay taxes wasn't based on any facts.