Here's what I probably should have posted earlier in the week, instead of this. Googling for "collaborative playlist" yields a number of online playlist sharing services. None of them make it easy to collaboratively build a playlist, but they're interesting enough that it's worth pointing to them anyway. Here's a roundup from DJ Alchemi.
The most interesting service here is probably webjay, wherein users create playlists that typically link to direct (and legal) downloads all over the web. Webjay will keep you in loads of new music for a long time. Alas, Webjay's model isn't that great for compiling mixes that include the music you've ripped from your CD collection. Also, users can copy links from each other's playlists, but it's not easy to put up a playlist and ask the world to help you improve it.
Mixmatcher (currently down) does permit users to add tracks to each other's playlists, or edit the order of tracks, but doesn't allow track deletion --- even by the playlist owner. (Better not mis-click when you're adding tracks to your playlist.) Mixmatcher's also rather slow and flaky, and the track addition interface is a pain.
The remaining playlist sharing services appear to be structured around vanity (i.e., showing off your music preferences, and getting people to rate them) rather than collaboration (i.e., people helping each other find music). Blech.
Jon Udell has some thoughts on collaborative playlists as well.