I don't have my Apple TV yet, but I've been transfering TiVo recordings to iTunes in preparation. Here's how I do it:
* Use one of the freeware Tivo transfer utilties for Mac (search on VersionTracker or MacUpdaate) to bring over the .tivo/.mp2 files
* Use Toast 8 to encode them into H.264/MPEG-4, then import into iTunes
* Use AtomicParsley, etc. to set the Title, Episode Synopsis, Artwork, etc. for the shows
Note: if you use the TivoToGo feature of Toast 8 to bring over your shows, Toast will limit you to encoding to H.264 at iPod resolution (320x240) which will look awful on Apple TV
MacBook 2.0GHz (Late 2006), Power Mac G5 Dual 2.3GHz (Early 2005) Mac OS X (10.4.8) Previous Macs: PowerBook G4 12", iBook G3, Power Mac G3, PB 5300cs, LC 575