So, finally, I got Xgl running on my IBM ThinkPad R50 as well. The R50 has a Radeon Mobility 9000 (M9) based on the R250 chip. Xgl is a nice toy, really usefull are the exposé like window switcher and the over all speed gain when switching windows and desktops. Another hot feature are the ongoing animations while switching apps, in semi-transparent exposé view or while rotating the Desktop cube. A great surpise was, that the GPU stays colder than when running glxgears on normal X. At least, the fan of my ThinkPad remains on slow (and therefore quiet) speed.
Sorry, but I'm to lazy to take screenshots, there can be found thousands of them already in the net.