2.7.1 is now available, and should hopefully fix compatibility with 10.4 and maybe even 10.3.9:

http://theunarchiver.googlecode.com/files/TheUnarchiver2.7.1.zip

That is the only change. Anybody on a newer version does not need to update, and there is no update for the Mac App Store version, as it does not work on 10.4 in the first place.