Haven't seen it posted here yet (though may have missed it), so I thought I'd share...
As it turns out, even though TiVo's website states the TiVo Home Media Desktop software for Series 2 TiVOs for OS X doesn't support OS X 10.4, whatever was causing the problem has been fixed in the 10.4.2 update. The software works just fine now (iPhoto sharing and MP3 sharing - still no .m4a or .m4p, which I worked around via AirPort Express and a Keyspan remote).
Verified by many, many others on the TiVo forums, too.
Hurray!