ehm, what would it be good for on such tiny display?
Plus it all looks well and good, but I have had nothing but problems with Desktop Linux when using compositing (though granted, I haven't used it a great deal on Ubuntu but Mandriva). I have always ended up with it turned off to avoid video tearing, complete lack of video acceleration, instability, you name it. It may be nVidia at fault but that does not alter the fact it was not usable. That is something which does not happen on Windows btw. So even on an environment where it SHOULD work (minimal difference in battery life on a laptop) it causes problems. I'm not sure I would want a dirty hack to get it on N900, it has perfectly reasonable transitions as it stands without wasting RAM and battery power.