Re: GTX 580 dual screen weird issue, HELP!
You can use MSI Afterburner to drop the 3D clocks to minimum for the desktop. Just crank 'em back up for gaming. Along with the quirk I noted above, that puts my 570 around the desktop power usage of my old 260. Before the system cycles through the screen power save, idle desktop usage is significantly higher, I think closer to 50W than 30W.
Yes, they say it needs two screens of same model / rez / dvi clocks to use the lower power gfx mode.
I hope the next gen cards bring some power savings.