Good iPad piano software?
Apple's Garageband for iPad is $4.99 and looks like it should fit the bill pretty well. It also has a whole bunch of other instruments and functionality that may or may not be useful for you.

I haven't used either, but Virtuoso Piano (Free) and Pianist Pro ($4.99) look like good bets, based on a quick search and the ratings. Pianist Pro was featured in an iPad commercial, which is probably a pretty good endorsement on its own.