Oh, come on. I used Nouveau a year ago on my Mac Mini (which I subsequently sold) with an NV40 based 9000-series IGP (can't remember if it was a 9400M or 9600M). You're telling me that Canonical has no reservations about completely turning around the desktop experience and confusing/shocking thousands of users with the crashy, counterintuitive Unity, but they're too conservative to ship a FOSS driver that worked fine with Compiz a year ago? Huh?
Maybe they should only enable nouveau for certain chipsets; I'd be fine with that. No doubt it's probably way too experimental to run on Fermi. And I doubt it's up to the task of doing heavy 3d as you'd find in Unigine or Trine (no Mesa driver is, for that matter). But Gallium3d drivers have long been proven to have the technical implementation needed to run compositing window managers very well. Nouveau really isn't an exception to this guideline at all, especially if you're running previous-generation Nvidia hardware.
This is disappointing. They are shipping r600g, though, aren't they? AREN'T THEY?