Not sure if it is a kernel issue or user-space. Truth is probably somewhere in the middle. It popped up moving to 3.8-rc1 using nouveau. Using nvidia's driver works fine. With nouveau, after entering login credentials in lightDM the user session does not start and I am back at the lightDM login screen. This is shown in the syslog:
Jan 5 11:28:34 linux-e6410-1 rtkit-daemon[1888]: Successfully made thread 4920 of process 4920 (n/a) owned by '110' high priority at nice level -11. Jan 5 11:28:34 linux-e6410-1 rtkit-daemon[1888]: Supervising 9 threads of 3 processes of 2 users. Jan 5 11:28:34 linux-e6410-1 pulseaudio[4920]: [pulseaudio] pid.c: Daemon already running. Jan 5 11:28:53 linux-e6410-1 pulseaudio[2767]: [pulseaudio] x11wrap.c: XOpenDisplay() failed Jan 5 11:28:53 linux-e6410-1 pulseaudio[2767]: [pulseaudio] module.c: Failed to load module "module-x11-publish" (argument: "display=:0"): initialization failed. Jan 5 11:28:54 linux-e6410-1 kernel: [ 2454.499025] nouveau E[ 4780] failed to idle channel 0xcccc0000
I can start sessions for Xfce and 'Gnome Classic (without Effects)' so I suspect compiz to fail. Did not try xcompmgr.
Gr. AvS