I have definitely thought about this and would love to spend some time on it, although my main focus of my development time in the evenings is on the iPad app.
A friend of mine tested some of the core granular synthesis code (minus all of the interface, etc.) on an iPhone 4 and found that the performance is extremely limited. I think he said he could get about 6 grains before the frame-rate of his app started to drop - to be fair, the app he is developing is also doing some pretty heavy graphics computation as well.
Anyway, it's an interesting idea. Maybe someday!