Disklavier

For another recommendation, I'm using the USB Uno MIDI interface from
M-Audio with HCX on my iPad 1 (with the camera connection kit).  It
works very well and can be had for around $36:

http://www.m-audio.com/products/en_au/Uno.html

My piano has a Mark III.  The benefit of using the USB Uno is that it
also works with a laptop (another platform where HCX is available).

One downside is that it is powered from the USB interface, so it will
reduce the battery time on your iPad.

John