Shawn,
I finally found the solution!
It was my Guitar Hero controller which was causing the problem! No matter to which USB port I connect it... no more monitor poweroff while this device is connected. I guess that this is a bug in the Microsoft XBOX 360 driver which is required to use this device. I'm already running the latest 1.1 version so I'll just have to wait for a new version to come out.
So to troubleshoot monitor (and probably other ACPI related) issues it helps to disconnect your USB devices. Running in safe mode doesn't help, some drivers are still being loaded. This isn't illogical as you still want to use your keyboard and mouse.
regards,
Peekstra.