Are there any decent physics engines for Android?

I have done an implementation of the APE engine now using (mostly) fixed point math and it is without garbage collection. Its a pretty nice engine to do stuff with constraints but seems less suited for really rigid bodies.

http://code.google.com/p/ape-physics-for-android/

You could also look into the NDK port of Chipmunk (download link on youtube page)

http://www.youtube.com/watch?v=_i_GFjfmLTc

I don't know why Dalvik would reject classes unless they were obfuscated with some tool. Did you try recompiling Box2d from source?

The only other Java lib I've seen is http://www.cokeandcode.com/phys2d/

I am getting good results with the Java version of APE using Fixed Point math rather than floats.

http://www.cove.org/ape/