I am in the process of uploading a GCC 4.7 build with posix threading (this means all the C++11 multithreading functionality in <thread> and <mutex> should work).
This build is "experimental", mostly because it's ABI incompatible with my other builds. Every C++ and some C applications will implicitly depend on the libwinpthread dll. Don't worry, it won't eat your computer. One more DLL won't kill you.
I (and I imagine Kai) will be very interested in performance and general usability information.