About half the time when the screen begins to dim to show that the screensaver is about to activate, the dimming cannot be cancelled by a mouse movement or key press. Only once the dimming has finished will the password dialog appear.
I can confirm that this problem is present on Fedora 13 and 14 on my Inspiron 8600 Laptop. In my case it is not possible MOST of the time to stop the screen saver once the dimming begins. Occasionally it works but mostly it fails.
Also, I think this may be a dupe of bug 612620
Good one! *** This bug has been marked as a duplicate of bug 612620 ***