I was reading some hand histories from gank on p5's and he never raises when he is heads up on the button. Most people think raising on the button is the best play because your getting more chips in with position.

gank said he limps on the button because :

it allows you to control your opponent much more and it is way to easy to get reraised and have to fold when you raise. many top pros limp on button heads up at the end of a tourney. It disguises strong hands and lets you get the reraise in as opposed to the raise.
when do you think limping instead of raising is the best play (and vise versa)