Ubuntu gdm problem solved.

I ran into this shortly after updating 9.10, got the black box in place of login screen, following log messages in /var/log/gdm/:0-greeter.log

(gnome-settings-daemon:3419): GLib-CRITICAL **: g_propagate_error: assertion `src != NULL' failed

(gnome-settings-daemon:3419): GLib-CRITICAL **: g_propagate_error: assertion `src != NULL' failed
Window manager warning: Failed to read saved session file /var/lib/gdm/.config/metacity/sessions/10a9a99fd3e4ce7c9b128608721774271800000034130008.ms: Failed to open file '/var/lib/gdm/.config/metacity/sessions/10a9a99fd3e4ce7c9b128608721774271800000034130008.ms': No such file or directory

/etc/motd told me system load was over 5, saw 4-5 gdm processes each eating 15-27% cpu.

I figured it was a faulty update, possibly of my evil binary nvidia driver. Turns out /tmp was full! Clearing it solved the problem.