If your cursor is grabbed by qemu/KVM (or I'm guessing other apps), then the screensaver still activates. Which is a bit unexpected...
Looks like a duplicate of http://bugzilla.redhat.com/show_bug.cgi?id=300321
This bug appears to have been reported against 'rawhide' during the Fedora 10 development cycle. Changing version to '10'. More information and reason for this action is here: http://fedoraproject.org/wiki/BugZappers/HouseKeeping
*** This bug has been marked as a duplicate of bug 300321 ***