Comment 27 for bug 777493

Also happend to me at Natty->Oneiric upgrade.

$ sudo lshw -C display
  *-display
       description: VGA compatible controller
       product: G96 [GeForce 9500 GT]
       vendor: nVidia Corporation
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a1
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
       configuration: driver=nouveau latency=0
       resources: irq:16 memory:fa000000-faffffff memory:c0000000-dfffffff memory:f8000000-f9ffffff ioport:df00(size=128) memory:fbf80000-fbffffff

This tells me that something is really broken, and I cannot use compositing in kwin either.