Newton's third law, you a fan of it? If you said yes, boy do I have the game for you. Osmos is a game that originally came out last year for the pc, but was recently ported to the iPhone. The basic idea is that you are a bubble that must eat smaller bubbles to grow and win the round. However, in order to move you have shoot small bubbles in the opposite direction of where you want to go. For every action, there is in equal and opposite reaction. I told you that you'd like this game.
The game has eight different game variants. Each providing a different kind of challenge. My only gripe with the game is that some of the levels that have you orbiting around giant bubbles are needlessly difficult. Still, I suggest you give this game a try.