I've tried brew, port & fink to no avail.
I'm still waiting for my build to finish, but i686-apple-darwin11-llvm-gcc-4.2 seems to be handling the gcc 4.7 source fine.
[1] http://gcc.gnu.org/install/build.html
I don't know what other versions of llvm-gcc are shipped with Mac OS X
I've tried brew, port & fink to no avail.