I just purchased Cubasis, and at long last, I found an iPad app that functions like I want. Its built-in instruments (both sampler and mini synth), MIDI input/output, interapp audio capabilities (i.e. using with Sampletank), audiobus capabilities (use with Thor), effects, etc are top notch and function wonderfully on my iPad 2. I never knew MIDI/Audio routing could be so damn simple!
That said, there is one thing missing that I think would be essential - a bit-crusher/sample-rate reducer. Why essential? If you used one on drums to try and simulate the output of an MPC 60 (12-bits, 32000 sample rate reduction), you'd know how that thickens the drums and makes them pop in a mix while staying set in a mix.
I'm not aware of any bit crushers/sample rate reducers on the iOS market that can be used as an Audiobus effect, but I'd much rather do it from within the Cubasis app. I hope this can become a reality. Cheers.