Is there any physics engine SDK for iPhone/iPod touch and iPad?
The Chipmunk engine is written in C, and they've even provided an Objective-C wrapper.
The Box2D engine is written in C++, and so it can be compiled/linked into an iPhone OS project by telling Xcode to treat it as Objective-C++.
Both are excellent. (I've only extensively used Box2D, but I've heard great things about Chipmunk too.)