Display will not sleep

I have a new issue with my Mac Pro (3,1 - 2008), it just popped up (and I don't think it was caused by 10.6.3, I remember this working correctly after it was installed)

I have two monitors connected via DVI to a flashed XFX 4870. The 4870 has never given me any trouble.

In the Energy Saver settings, I usually set the machine to never sleep, and the displays to sleep after 5 minutes. This used to work great, never had any trouble with it, ever. Now, the displays don't go to sleep after 5 minutes, they just stay on with a screensaver.

I tired the following:

1) Clicked "restore defaults" button in Energy Saver control panel, then set my setting the way I usually do.

2) Set a hotcorner to put the displays to sleep. This worked once or twice, then the hotcorner stopped working.

3) Rebooted a bunch of times.

I have not yet reset PRAM or SMC because this is a monitor/display sleep issue, not a machine sleep issue (machine sleeps just fine).

I have a full SuperDuper backup of my 10.6.2 system and when I boot into that, display sleep works just fine. But I could *swear* that display sleep was working OK in 10.6.3 before this issue happened. I think...

Any ideas? Can I blow away some plist files to reset this? Someone suggested running Repair Permissions...has anyone had success with that?