HDMI v. DVI Power..

I want to use HDMI (not that it matters any vs. DVI), but it doesn't react to "shut down monitor after 20 minutes" in Win7 (nor in Vista before I upgraded to 7).

I have a Radian 4750 video card, ASUS P5Q Turbo Pro MoBo. 8 GB RAM, Quad Intel CPU. The DVI power behavior into this monitor works fine. My monitor is a Viewsonic VX2640w. But the HDMI simply won't force the monitor to sleep.

Any ideas, anyone? Also, for PC's, IS there a video advantage in HDMI vs. DVI? I find varying opinions on the web.

I should have added, when the HDMI cable is in use, the 20 minutes expire from the power setting, the video card goes to sleep, but never the monitor.