Clang-on-llvm will ultimately be a big improvement over GCC in a lot of respects (speed, ease-of-use), but it seems like it's currently very immature. Perhaps the BSD developers are doing this in order to get the development resources put towards clang in hopes that it will get the development resources necessary to become a real replacement for GCC. Perhaps it will also help them make their code more portable by having to remove GCC extensions from it.
No announcement yet.
LLVM/Clang Replacing GCC In FreeBSD Base