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It also does a hell of a lot more than XGL/AIGLX.
Such as? The only real difference I've heard so far are pixel shaders, and I don't see why these couldn't be added to XGL. One thing we know is that the open-source desktop has developed faster than proprietary ones, so we'll see where we'll be when Vista comes out, and then a year later...
In any case this is quite off-topic. I'm personally happy to see cooperation between the two projects.