Just wanted to introduce another port of the great Box2D engine for the flash world. It's called 'motor physics' and based on Box2DLite. The most challenging aspect was to make it as efficient as possible, because even AS3 is still awful slow compared to c. As a result many parts were rewritten and custom collision code added (also based on SAT). By now the code is that much optimized (and sadly unreadable) that in some situations rendering takes longer than the computation itself ;).
Here is a demo: http://lab.polygonal.de/2007/07/16/updates-2/
(brute-force, 30 iterations, stepsize .01)
Some of the features I'm working on are swept-collisions for fast moving objects, a particle system and broadphase collision schemes. I'm planning to release it soon this year.
And Erin thanks for the improvements ! It's good to see you are still enhancing it :)