I did this with Hulu (the free website), and it works "ok", with the following drawbacks:
-You have to use the "screencasting" trick in conjunction with AirFlick, which means you are just mirroring your computer's screen on your apple tv. Instructions are here:
http://www.tuaw.com/2010/12/22/screencast-your-mac-to-apple-tv-over-airplay/
-As far as I can tell, you can screencast the video from your mac, but not the Audio, making this somewhat useless at the moment for watching internet video.
-The frame rate is something less than 30 fps, so the video looks choppy.
-the screencast is delayed about 15 seconds or so compared to what is happening on your mac's screen.