When you purchase a song on iTunes, you are authorized to play back that song (in the purchased AAC format, anyway) on no more than 5 computers. Once you've authorized playback on 5 computers, your only choice is to skirt the DRM by doing the burn/rip dance or de-authorize your previously authorized computers. But what if you don't have access to those computers anymore to de-auth them? Well once a year (officially speaking), you can ask Apple (via iTunes support email or from your account page in the iTunes Store) to de-authorize all of your systems and start from scratch, giving you 5 free auth slots back. In many cases, even if you've used your "once a year" allotment, emailing iTunes support will get you another reset anyway, but it's not a guarantee. This is a much more common issue in school or work environments where users regularly get moved to new and/or different computers for a variety of reasons and the last thing on their mind when that happens is de-authorizing their iTunes purchases on their old system. And trust me, it's the last thing on the IT department's mind as well.
On your Mac, the iTunes authorization info itself is stored in an invisible folder located in /Users/Shared/SC Info/ . It seems the key to unlimited auth and de-auths is to remove then restore that folder. If you have a need to do this regularly, Firblitz has whipped up an app to make it easier. iTude is an AppleScript application that backups us the SC Info folder while you de-authorize iTunes and then restores it, which theoretically means you don't waste an authorization slot for that computer.
I haven't personally tried this out, since I really don't have a need for it, and the author himself warns that you should use it at your own risk, but if you have a need for it, it's there for the taking - until Apple rolls out the next iTunes update, which will probably break it :)