Bump.
Same exact problem here, but with Windows XP SP2.
I also experienced the problem with Vista 32 bit, but never bothered to fix it. That's a different story altogether.
Here's the process I go through when I notice that the monitor isn't turning off after set time of 15 minutes.
1. Set it to turn off in 1 minute. Wait 1 minute.
2. The monitor will always shut off in 1 minute.
3. Set it back to 15 minutes.
4. Monitor turns off in 15 minutes, until it stops doing it again (usually takes a couple of days until it quits going to standby).
5. Go to step 1.
E6600
Gigabyte 965P-Dq6
2 GB Crucial Ballistix RAM
BFG 8800 GT 512 MB latest WHQL drivers
Creative Sound Blaster Xtreme Gamer with latest drivers
Seagate 320 GB, Maxtor 500 GB, WDC 750 GB
Dell usb keyboard, Logitech G5 v2, samsung 2510 laser printer usb
Windows XP SP2 with every update (could be the issue?

)
EDIT: I realize the problem could be that the Logitech G5 is very sensitive so it has a greater chance of causing the monitor to not go to standby in 15 minutes, but when I go through the steps, the monitor ALWAYS shuts off after a period of inactivity. In addition, the monitor NEVER shuts down during the time that it is not going into standby after 15 minutes (and believe me I've let it go for a whole week and it has never gone into standby). Therefore I think it's safe to rule out the mouse being the source of the problem.
Since changing the Windows properties to 1 minute and back to 15 minutes, I am thinking it is a simple Windows glitch. Maybe a certain Windows update triggered it?
Anybody else ever experienced this?