Saw some discussion on this in another thread (about the Jabber server), and I figure I'd mention I'm working on something like this. I'm wanting to keep it simple (accounts, high-scores, achievements and matchmaking only) whilst I develop the stack (easy to use library connects to low-footprint QT app/daemon connects via HTTP to server). I don't really want to say too much since it's so early and might never be finished (I'll release the source in this case), just wanted to mention I'm working on something...
I've been calling it "Panda Network", since the name is close to Pandora and pretty cute. Open to ideas and suggestions, but I want to keep this pretty small initially.