I recently did a quick demo on my site of Box2D (Box2DFlashAS3) with source code. Its basically just the HelloWorld example from the v2.1a distribution, but with a few tweaks. The most notable is that my version does not require a Flash FLA file to compile and run, its pure AS3. Simple and a good base to build on:

http://savagelook.com/blog/code/box2dflashas3-fun