I come across the same problem.
No matter which variety of driver I use for my monitor or graphics, it won't allow my monitor to power off!
Graphics: Gainward Ti4200 - Golden Sample
Monitor: Hansol 710D 17"
OS: Win98 SE
Monitor power-off setting: 20 minutes
Both are about 2-3 months old, but the problem started when I installed the Gainward.
Oh good old Voodoo3 2000....*sigh*