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Cairo, xcompmgr and Xorg not integrated? What the hell are you talking about? They already are, for heaven's sake!
xcompmgr is not meant to be a finished product, it's a testbed. The whole idea of the Composite-extension is to let the window manager take care of it, like Luminocity does. If that's not integration, I don't understand what it is you want integrated.
The problem is, this is very new technology in the X11 world. It's much easier to do on Windows and OS X since they are free of the client-server model, but we have to take different approaches. And once the Composite extension in X.org matures, there are no obstacles. This is technology that works here, now, today. It will certainly be stable long before 2007. A lot of people use it with great stability right now.
The non-accelerated drivers are not an issue, there have long been software-rendering support for OpenGL on Linux. It's much slower, so people without hardware support would probably avoid it, but the vast majority of Linux users using NVIDIA, ATI and Intel cards will be able to run this very smoothly.
Once the Luminocity stuff gets implemented in Metactiy, compositing will be a non-issue for Linux users as well. And I can't wait. I just tried a Mac OS X desktop the other day, it's REALLY useful! Especially the Exposť window management. That thing rocks! A very good example of how this stuff can increase productivity and provide a much more effective GUI environment.