In the past, iTunes music sync only worked in one direction: from the computer to the iPod. The rationale for iTunes only sending the music in one direction is one of piracy…the rationale is that if you can take the music back off, you can share it with your friends. Stealing! But what you’re trying to do isn’t stealing…it’s simply about moving music you already own from the old computer to the new computer, which is a backup process. Now with the newest version of iTunes and the newest iPod firmware, you should be able to resync the content from your iPod back to another computer, so long as that computer is one of the five “authorized” computers for playing that content….and so long as you’ve purchased that content on the iTunes store. More info here: http://www.apple.com/itunes/sync/transfer.html

That probably won’t work for all of the content on your iPod; thankfully there’s another way. The program Senuti (that’s iTunes backwards) not only can move the music from your iPod to your Mac, it can automatically add it to your iTunes library at the same time. You can get it here: http://www.fadingred.org/senuti/