Some 'pretended' the same thing with the then (snarkily) titled King Obama in 2009 and his initial refusal of releasing his birth certificate. That was a first re a seated U.S. President.

When it was released (2 years later!) it revealed nothing exciting. http://www.cnn.com/2011/POLITICS/04/27/obama.birth.certificate/index.html

I suspect that will be the same result with the tax returns.