I had the same problem where the hot corners worked for everything but the screensaver, and this appeared after I was forced to reboot when the MBP locked up.

I tried the "delete the .plist" file solution, even though I'd lose all of my Dock settings, but it didn't fix it.

I used the Disk Utility to repair the permissions on the hard drive (select the disk on the left pane, and click the "Repair permissions" button at the bottom of the right pane), and then rebooted again.

This got my screensaver working again...and I didn't have to sacrifice all of my Dock settings.

I had the same problem where the hot corners worked for everything but the screensaver, and this appeared after I was forced to reboot when the MacBook Pro locked up.

I tried the "delete the .plist" file solution, even though I'd lose all of my Dock settings, but it didn't fix it.

I used the Disk Utility to repair the permissions on the hard drive (select the disk on the left pane, and click the "Repair permissions" button at the bottom of the right pane), and then rebooted again.

This got my screensaver working again...and I didn't have to sacrifice all of my Dock settings.