Probably not exactly what you're looking for... But I always liked the Pocket Piano from Critter and Guitari... There is a midi version now: http://www.critterandguitari.com/products/pocket-piano
I always thought it had a sort of "chippy" sound, and they are a lot of fun!
And a good app to try is "Squaresynth", I really like the sounds in it. You can find it by searching under the iPHONE apps, but it works on the iPad as well.
The Pocket Piano seems pretty damn interesting. I'll have to look into that further.
Squaresynth is actually what I'm using right now. I love the sound and it's simple controls, but I can't get any of the functions on my keytar to work with it. At least being able to control the volume/gain from my keytar would be nice...