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The final open source stack will look something like this:
1) framebuffer/DRI device drivers - both will be required
2) mesa with EGL. this is being designed right now.
3) XGL the X sever based on OpenGL.
4) luminosity or some other window manager.
5) Cairo running glitz
Nvidia/ATI will have their own versions of 1/2.
Right now XGL is an X server running inside another X server. You are only seeing technology demos. No one has a standalone OpenGL stack working to run XGL on yet.