Not quite what Steve said in the keynote. Up to fiveTVs can stream protected content from an iTunes library, but only one can sync content to its local drive if I remember correctly.
Apple TV streams as well as it syncs, so you can pair up to five additional computers and let friends and family stream their iTunes libraries to your TV. Apple TV stores up to 50 hours of video, ready to watch when you are. And if iTunes is still syncing what you want to see or if you dont want to sync at all you can watch a stream from your computer right away.
I use a MacPro to sync 5 atv's. I stream 9 atv's from this MacPro. You can only sync 5 at a time. I am using new Airport Extreme. It is incredible to play the same movie on 5 atv's at the same time and in different places within this same movie. Over five going at once starts stuttering/freezing.