This does sound a little cool, but I can not help but feel they are limiting themselves by just supporting Windows. There are enough games on Mac and Linux to justify that they offer support for them, especially if emulators like Dosbox are being taken into account. I at least hope you can convince them to suport the OpenGL renderer at least.
Do not get me wrong, this is cool news, even if it would probably suck to much bandwidth for me to try it out personally.
EDIT: It looks like there have been others with similar views, and have actually been trying to reverse engineer the protocol. I still think they should extend official, full support, to the mentioned platforms though.
http://en.wikipedia.org/wiki/Xfire#Thir ... ifications