Many users assume that to physically copy your iTunes Music Store (iTMS) AAC songs onto other computers, you need to first make AIFF copies and then recompress them into MP3 or unprotected AAC files. Not true. A little understood feature of iTunes is that it will allow you to directly transfer protected AAC files to all five of your iTunes authorized computers.
First, copy your files as a Data CD using Burning preferences. You can copy anything in your library, including the iTMS restricted music, in their original format. Then, load the burned Data disk onto another computer with iTunes. iTunes will want to verify the receiving computer is one of your five authorized computers under your iTMS account. Once authorized, the new computer will load the iTMS songs, along with everything else. You can then play and burn just like you can on the original computer.
This is a great way to have playable backups of iTMS files, and also to have a copy in your work computer - in case you don't bring your iPod.
[robg adds: This is probably an obvious hint, but I hadn't thought about it -- we have a network at home, so I've copied our purchases that way. For work, I take my iPod, so I've never tried to transfer an iTMS purchase to that machine...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040510193418462