I have a feeling that KWin's work has been instrumental in all of this. The fact that they are switching to cmake makes this pretty obvious.
I wonder if compiz will be replacing KWin in the future? Perhaps it can replace Metacity too. That would be a really great thing for the linux community. Or at least this modularization should make it much easier to share code between the projects.
Granted, it would be trickier to share with Metacity since it doesn't use C++.
Considering how many distos dump metacity for compiz, it would make perfect sense for GNOME to dump it too, and for all of us to rally around one window manager.