You can't transfer MP3s back from your iPod to your computer when they're in iTunes format (that is, in a format that you can play from your iPod when using it as a portable music player). However, you can transfer regular data files (including MP3s) back from your iPod to your computer if you originally just dragged and dropped them into the iPod's hard drive. Unfortunately, you can't play MP3s from the iPod if you've added them this way.

This leads to an ingenious solution to the space-saving problem that I mentioned below: buy a 30 GB iPod, put 15 GB of MP3s on it in iTunes format, and then store the same 15 GB of MP3s on the iPod's hard drive as regular data files. That way, you've freed up your computer's hard drive, you can easily transfer MP3s in either direction, and you've still got 15 GB of playable music on your iPod (which translates to something like 3,000 songs, which should be enough for anyone). Wish I'd thought of that earlier.