By the way, Magic SysRq seems to work. That is, 'Alt-SysRq k' kills the locked up X session. It is not always that gdm3 recovers correctly and if X is respawned it might perform slow and unstable.
Another option is to use Magic SysRq to reboot as gently as possible without removing your hads from the keyboard (something along the lines of 'e i s u b'). //Makholm