I have the screensaver set to require a password to unlock. it's "casual" security agaisnt my 8 year old curiosity and my toddler's facisnation with keyboards.
On Friday I came back from work, the monitor was in power save but the screensaver hadn't kicked in.

This had happened a few time sin the past, but this is my brand new Spring 2007 install, so I would think it's mostly "clean". The screensaver is simply a blank screen if it matter.
Interestingly enough we had a 5 minute power outage that day. The UPS kicked in so the machine never rebooted. I don't know if it has anything to so with it.

I went in the sreen saver settings and checked that it was set properly (which it was) but that didn;t solve the problem

It was time for my monthly system maintenance anyway, so I shut the system down, did my maintenance (clean up fan filters, etc...) then rebooted and the problem is gone.
I'm guessing restarting the X server would have been enough, but it's just a guess, I'll try it next time this happens.

any ideas of why the screensaver stopped kicking in?