This seems contrary to what most people want, but I've got a somewhat different setup.
I have my desktop computer hooked up to my HDTV as a second screen - I don't always have it turned on in that configuration. But when I do, I have XBMC running on the TV while I do work on my normal monitor.
Sometimes I'll stop working on the computer for a brief period, but fail to bring XBMC to the focus, because I don't need to think about it since it's already running full screen on my TV anyhow. But if I do that, after so many minutes, my power settings kick in and then shut off all my displays - my TV along with it.
The perfect solution for me would be to allow my regular monitor to shut off but keep my HDMI port on my ATI card fully active... but failing that, I'd like to ensure that my displays stay on all the time, except when XBMC isn't running on my TV.