CinemaTech reports that "AppleTV users will be able to watch a selection of YouTube's most-viewed and most-discussed videos on AppleTV..." via Ars Technica and Wired.
I don't have an Apple TV box, but it doesn't seem all that easy so far. I can't tell how the ATVFiles browser plugin (plays video files without transcoding and syncing/streaming them) is different from the "A Series of Tubes" plug-in. To explore what is going on under the hood of the Apple TV, check out Apple TV Hacker and AwkwardTV Wiki.
Update: HD For Indies has further "Thoughts on HDTV DVD playback - AppleTV, uprezzing DVD player, or HD DVD player."
Update 2: On May 30 Steve Jobs demoed Flash on Apple TV which should be available as a free update in June; see Apple TV and YouTube, together at last and Macworld on All Things Digital.