I just got a second display for my iMac and am wondering if there is a way to put the second one to sleep without putting both displays to sleep. I occasionally use Ctrl+Shift+Eject to put both of them to sleep but if I'm watching a movie on one, it would be nice if I could make the other go dark without having to unplug it. That causes all my windows to get all screwed up. If not, I guess turning down the brightness all the way sort of works.
Check out DisableMonitor. It is a tool that allows you to change the resolution of a monitor and/or enable and disable it. It works with multiple displays, and it should fix your problem.