I've just added a port of the TestBed for the iPhone to the project. Most of the tests are in there, and you can use the touchscreen to interact with the environment or the accelerometer to change gravity.
There's a bit more information here - http://www.handcircus.com/2009/01/15/ip ... available/
I've tried to avoid changing the core Box2D code too much, I've created alternate versions of the Testbed code specifically for the iPhone (in Build/iPhone), but its still using the existing files for the Tests themselves. I had to make a couple of tweaks to b2Math.h - I hope this has not caused any issues elsewhere.
The only weird thing is that for some reason I'm having to manually define TARGET_OS_IPHONE in the project settings, even through it should be picked up when it compiles (so this causes errors when compiling for device if you don't have this manually defined). If anyone knows why this is, do let me know.
I hope this is of use to anyone playing with the iPhone