Ctrl+Alt+Backspace is the traditional Kill-X-Immediately shortcut (aka "DontZap"), but it's been removed from the X-server in recent versions. It's not enabled by default, but there are various methods to enable it.
If you can login on the text console, or if you've installed the SSH server on the machine, you can use those methods to enable DontZap, or you can use one of these commands to restart GDM:
sudo /etc/init.d/gdm restart
sudo restart gdm
Another option is the Magic SysRq keys. Be very careful with them, but if necessary, they may be useful. The DontZap wiki article adds this:
For anyone missing the ability to kill the X-server in emergency, may I point out Alt+SysRq+R, followed by Ctrl+Alt+F1 (the first puts the keyboard back into Raw mode, i.e. outside the control of the X-server; the latter switches Virtual Terminal.) Or you can use Alt+SysRq+K to kill the current session.
From your Ubuntu Forums post, it really sounds like you've loaded an incompatible graphics driver (or maybe just an incompatible setting). Use bryan's suggestion to test the graphics hardware by booting to a LiveCD; if that works you can focus on uninstalling or reconfiguring the broken graphics driver.