Does anybody use the Ipad as a sound module? For example with a synth app or the sampletank app ?

Fran,
I have the midi adapter and I have used it as a dedicated sound source. I ran the output from my Yamaha psrs910 last year into the iPad for a separate bass output. Worked great.

Yes I have listened to some demos of sampletank and garageband for the Ipad and it sound very good. I don't have a Ipad yet but I am thinking of getting one. Thanks for your answer.

Has anyone tested I/O latency for iPad music apps?

Latency in computers is always a function of how much work the computer is doing. Play simple one note lines on a simple sound, it's going to be pretty good.

But stack two or three sounds up on every note, play fast repeated chords... now how's it doing?!