I've been looking for this same ability for ages (I dropped my external drive with all my music, and had to re-make my libraray off my iPod), and could never find anything that worked qute right. Turns out one of the programs in the list lunarboy linked to works exactly right: Yami-Pod
It lets you specify a directory on your computer where your current music is, then looks through these songs to make the list. It takes a little while (esp. if you have a large library), but is well worth it!
