Since I started using Natty I experience a strange situation at times. When I leave my system idle for 30+ mins., the screensaver Ant Spotlight remains active for specified time, then the monitor turns off after a while. Later when I am back to work on moving mouse/pressing keys on keyboard monitor powers on but I am greeted with a stuck up Ant in the screen. I won't be able to access open applications in any way. No key/mouse click works. I tried going to terminal with Ctrl+Alt+F1 and killing gnome-screensav but the Ant doesn't budge. Only way out I take is to log out with Ctrl+Alt+Backspace which works causing to lose my unsaved work.
SnapOverflow
I have the same problem on mine... does the same thing with GLMatrix.
At first I thought the problem was maybe due to using encrypted home directory, and since that was causing me grief in other ways I re-installed sans encryption. Still have the same problem.
Only fix so far that I've found is to turn off the screensaver.
The trick that helped for me was to go to the text-only terminal and then immediately back to the GUI, that is: first press CTRL+ALT+F1 and then CTRL+ALT+F7. This does not do anything about the cause of the problem but allows you to start using the GUI again.