Updated from May 2011: French developer Motion Boutique has created a physics engine for After Effects.
According to the company’s website, “Newton interprets each 2D layer as a rigid body in a real environment. Once simulation is completed, animation is recreated in After Effects with standard keyframes.”
The scenes in the demo are comparatively simple – mainly falling beads and type effects – but the fact that they’re happening in After Effects at all is the interesting part.
Newton is available for After Effects CS5 and above, on Windows and Mac OS X. It costs €179 (around $250).
Read an article on Newton from one of the beta testers (French only)