My Gainward GF4ti200 http://www.gainward.com/c-1.html?id=18 has a standard monitor output, an S-video output, and a DVI connector next to it (came from Newegg for about $180). I hadn't tried it before until now, but I can do exactly what you're describing with it. Pretty cool really...

Under one tab, I can tell it to use the "second monitor" to clone the main one. Elsewhere I can turn the second monitor on and off. I'm running the HDTV off the S-video output, but I really have to beleive the DVI will work as well. There are some built-in "detect displays" features that should handle that.