I can think of a couple of ways to get out of something like that... although I've never had such a problem.
Normally, you can hit ALT+F2 and you'll get the "RUN" dialog where you can type xterm and kill the process using your normal procedures.
If nothing else works, you can always hit CTRL+ALT+Backspace. This will kill your X window session though... You'll need to log back in. Unsaved work will be lost... you've been warned.