Monitors will not standby or power down after specified idle perio

Anybody else having this power scheme related problem? If so, know the
solution? Please let me know.

Here are the details:

In my Vista power schemes, screen saver is set to turn on after 15 minutes,
monitors are set to go into standby (power down) mode after 20 minutes.
After an idle time of 15 minutes, the screensaver does reliably always turn
on, however, the monitors never power down or go into standby. There are no
processes running and no activity on the machine during this time. I have
tried setting and resetting the power scheme options, have tried
experimenting with different idle times, screen savers, multiple reboots,
logins, etc, all to no avail. The monitors will never go into standby no
matter what I do. Not sure if this is a Vista problem, or if it might also
be a display driver thing or something...?

Here are some of my system stats, in case they are relevant:
Vista Business, upgraded a WinXP system (no clean install)
Core2Duo, with Intel board
nVidia 7600 dual DVI card with dual monitors, using nVidia's latest
drivers (7/27)

Not sure what's going on. All of this stuff worked fine for years on WinXP.

Any help or suggestions would be greatly appreciated.