Greg was nice enough to point me to the PathScale compiler suite: a high performance compiler suite for AMD64. Pathscale's compiler is based on Open64, but has reportedly updated its C/C++ compiler frontends to a recent GCC (as opposed to the current Open64 derivative compilers).
OpenScale is lead by Fred Chow, an ex-SGI developer that created WHIRL, and later Pro64.
So as a starting point for a Managed C++ compiler, PathScale sources might be a better option.
This product shows one of the splits that people were afraid of to circumvent the GPL: the front-ends which are fairly complex (C++) has been split out from the backend and a proprietary highly optimizing compiler has been developed for the AMD64. It was just a matter of time before someone did this though.