Box2D is quite fun.
I'm beginning the process of ripping out the innards of the new Closure game (by beginning I mean I'm not touching the game part till after PAX so I don't not have a demo) and shoving Box2D's physicsy goodness inside.
Problem is it has no bitmap collision primitive (vital to Closure's design). So I'm hacking one in. So far I got circles to work with bitmap shapes, and I tested it with a couple hundred little balls rolling around on a bitmap shape. It's fun.
Next up is to get polygons working with the bitmap shape, which is gonna be a much much more difficult task