I as well have found no way to turn off the iMac's display!
My setup is Mini DVI video adapter, S-Video cable, 27" flat screen TV with Overscan on in Options to fill screen. Using extended desktop I drag the DVD Viewer and Control panel to the TV's display. I dislike using the mirroring feature, because it fouls the iMac's resolution when watching a movie on the TV and working on the iMac's display.
The best I have been able to come up with is:
1. To somewhat dimm the iMac's display I hide the dock, put away (yellow button) all open windows then press F9 to dimm all except the menu bar.
2. Just use a dark colored desktop image!
Capaho can you give us more details, that doesn't work for me the screensaver plays on both displays?
Greg I also use Shades and SleepDisplay there great, but they also dimm / sleep both iMac display and TV?
Dennis