на свежем сизифе
http://nouveau.freedesktop.org/wiki/TroubleShooting#Xorgfailstostartwith.22.28EE.29.5Bdrm.5Dfailedtoopendevice.22
1. Xorg fails to start with "(EE) [drm] failed to open device"

Did you miss the basic question "Are you using the latest code" above ? If you are using a 2.6.33 kernel from kernel.org , that nouveau code is not compatible with libdrm 2.4.18 or git. Either upgrade nouveau drm code (see InstallDRM) or use an old git version of libdrm and DDX, 2010-02-15 or older.

The problem can also happen the other way around, with a libdrm too old compared to the other components. If you built libdrm yourself, the system' libdrm could still be used to load the nouveau X11 driver. Just run the ldd command on nouveau_drv.so to check the libdrm and libdrm_nouveau libraries listed there are the ones you installed, and not the system ones.

Here is an example with libdrm compiled and installed to /usr/local:

    *

      $ ldd /usr/lib/xorg/modules/drivers/nouveau_drv.so
      ...
              libdrm_nouveau.so.1 => /usr/local/lib/libdrm_nouveau.so.1 (0x00007f3cca1bc000)
              libdrm.so.2 => /usr/local/lib/libdrm.so.2 (0x00007f3cc9c5c000)
      ...