Do you have a Microsoft wireless keyboard or mouse? If so, you can disable
the software/service that enables the extra function keys and get back your
screensaver. Apparently is prevents the screensaver from running.
There's another bug in Vista that prevents the screen saver from
ALWAYS working. I see it work, then it don't depending on which window
has the focus at the time. I have mine set to 10 minutes. Sometimes I
come back and see the screen saver, other times I come back after 20
minutes or more and there's the same application still showing full
screen and no screen saver active. XP did the same thing at times.
Vista adds a new wrinkle in that there's a third happening. Once in
awhile instead of a screen saver I'm back at the initial page when it
is just my long in and little thumbnail photo. I know the system
hasn't rebooted because I'm in the middle of rendering a video and
that is still chucking along.