For those of you who want physics in your game and would rather not keep switching your mind between objective-c and another language, you may want to check out Objective-Chipmunk; basically an objective-c wrapper around chipmunk with a few extras as I understand it. There's also some good tutorials on how to go about using it, here's the original post:
http://www.slembcke.net/forums/viewtopic.php?f=1&t=1118&start=0&st=0&sk=t&sd=a