I've added a new build system to Bullet CVS.
It's based on autoconf/Jam, and the platforms it should work on include Linux, Cygwin and MinGW/MSYS.
So to build Bullet with it, do "./configure && jam". Linux users probably have a package for jam (or boostjam) in their distro. Cygwin and MinGW/MSYS users can use http://www.minet.uni-jena.de/~res/jam.exe (a binary compiled by myself).
The system also contains facilities to generate VC projects. Those generated projects are in subdirectories of msvc/.
It would be nice if you'd try it out and let me know what you think of it or of any issues etc.