Thanks to Shaun for pointing out this new (and free) iPad app. Here’s the description:
iCSound gives you an opportunity to create you own music in a very creative way. You can actually “see” the sound from various musical instruments and pre-composed loops mixed together.
iCSound is very flexible and expandable, all the effects have their own unique parameters, which can be separately setup to be applied on either one sound loop or even whole composition.