A game I just released for the iPhone, Gopher Go Boom, uses Bullet Physics. The game is a labyrinth/exploding gopher mash up (
Video). The physics runs lightning fast - we were quite impressed. We're primarily using Bullet for the collision detection. The actual rigid body dynamics is fairly simple.
One of the tricks to getting this to work was to disable sleeping on the ball. Otherwise it goes to sleep, and the shifting gravity forces have no effect. We updated gravity in the accelerometer thread. Since we wanted the ball to roll, we had to use full 3d physics, rather than 2d.
Enjoy:
Video
iTunes Store Link ($0.99)