I'm poking around trying to find a good physics engine to use. Here are some resources I've found useful.
A possibly biased comparison chart.
A blog post on the subject.
See also: *drawlogic for pretty comprehensive coverage of the field.
The gist of it seems to be that, if you want a fully featured engine, go with something derived from box2d, which is the fully featrured open source 2d physics engine for c++. You have a few choices here. Box2DAS3 is the official port, which stays very close to the c++ API and implementation. Motor2 and Physaxe are both forks which are optimized for AS3 performance. But Physaxe is acutally written in HaXe, which is interoperable with AS3. APE is an older, action-script only alternative, but active development seems to have died some time ago.
Hm. My main requirements are that it have a community of users, and be under active development*. For this game in particular I'm less concerned about raw performance, since I don't anticipate handling more than 10-20 items at a time. So I will go with Box2dAS3. Perhaps for my own (speculative) titles I'll move towards Physaxe, though.
*This is the best quality filter I know of for open source projects.