Here is the current bullet physics integration library converted to an extension.
There are slight modifications to allow for (hopefully) cross platform use (tested on linux).
I wanted to test how easy it was to create and extension and I thought i would share my results.
Extension modeled from the terrain extension.
One needs to provide a bullet install and adjust cmake to point to the headers and possibly library location to link to.
See readme for install details.
Download:
http://www.mediafire.com/?7906s2nulrl9mti