After reading all over the net that it was impossible, I've figured out a way to use two USB headsets to record a podcast (or other vocal track) in GarageBand, requiring no extra hardware at all.
It's a little bit involved, so I created a blog page to describe the process.
[robg adds: After reviewing the above page, I think it's a bit more than "a little bit involved." :) I haven't tested this one at all, but for those of you looking to record via two USB headsets, this appears to be a solution...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060119113827587