This one was weird. I run my home machine on Gentoo 95% of the time and a while back the Gnome screensaver just stopped working. Usual problem; I'd made a few changes at the same time. So was it an update, a change, settings, solar flares?
After some playing and testing it was an update but one that I'd not followed the instructions on and had caused me some other problems. Python had been updated from 2.5 to 2.6 and I noticed a few things wouldn't compile and I had to re do the python modules to make them work.
Turns out there is a nice python-updater program that does the work for you. So give that a go if your screensaver just stops.