A bit of good news, a bit of bad news.
The good news is that I brought my bricked Apple TV back to life by blessing its Core Services folder. I then did a complete system restore, and then starting from scratch installed ATVFiles and the latest Perian and I can now play AVI files directly from the Apple TV interface. Woo hoo! They look fabulous and high def. (Take that Peter Svensson of the Associated Press.)
Also running: the latest beta of Joost, but with the SVG font issues other people have reported. (If you go back about 3 or 4 betas, Joost didn’t have the font issues. They’re working on fixing them now. If you’re having problems with Joost, make sure you copy both preferences files from an Intel Mac installation from ~/Library/Preferences (there are two) *and* also the Joost Application Support folder.)
The bad news? I connected my “good” Apple TV disk to my 733 G4 and now it won’t boot. Surely there will be a time when both units are up and running at the same time? Does anyone know a quick and easy “bless” solution without having to rebuild the entire disk from an image? Blessing Core Services didn’t work.
 For the curious, the way I bricked it was this. It was about 3 or 4 in the morning. I just got my youngest child back to sleep. Suddenly I had an Idea. (Never trust Ideas at 3 or 4 in the morning.) I decided to put a blank disk into my Apple TV, connect the good disk via USB and then press Menu-Down during boot. It was not a Good Idea. It was a Bad Idea. Hello Question Mark of Doom. The current bricking comes from having connected the other disk to a G4. Frack.