I don't have an HDTV. Just a 32" thingy I bought about 5 years ago, at most, 'cuz it was cheap.
My AppleTV is hooked up to it, running at 480i, the only possibility that worked on my TV. And it works just fine. I've watched a number of TV shows and movies, as well as listened to music from my iTunes library via my TV's speaker system. Plus, of course, seeing my various iPhoto stuff that we have synced.
And we rather enjoy watching YouTube on the AppleTV, if only because it's nice and big, as opposed to how crappy it looks when you scale up on your monitor.