A grab is when you type and type and click and click and nothing happens, yes? When that happens, I switch to a console and back (Ctrl-Alt-F1, Ctrl-Alt-F7 unless you're on Jaunty or F10) and usually that breaks it. If not, I switch back to the console and start hunting processes. Usually I get some innocent processes before I bag the bad guy but the damage tends to be pretty light.
If there's a keystroke that would save me the trouble I'd love to know it! Google doesn't seem to know how to release grab.