I’ve been playing with Google’s Android App Inventor site, and it is a lot of fun to mess about with. However, at the moment, it is unlikely to be useful for any kind of serious musical application building.
There is talk of being able to introduce extensions to App Inventor so you could do more with it. Things like using Pure data would be useful if you could integrate that into what App Inventor is about, and maybe that will come at some point, but at the moment musical applications will be somewhat limited and toy-like.
Still. Good fun to play with.