Just read an intriguing market study from Mike McGuire and Derek Slater predicting that by 2010, 25% of legal music purchases will be driven by taste sharing mechanisms, including (especially?) playlist sharing. The report was jointly commissioned through Harvard's Berkman Center and Gartner, so it trends towards the business/marketing end of things, but assuming the survey methodology is sound, points at the potential for another Web native form to emerge.
There's money to be made in them there playlists!
Of course Webjay's Lucas Gonze has been pushing, to good effect, on sharing of playlists that point to free streams. The McGuire and Slater paper documents a number of music file and playlist sharing mechanisms within commercial efforts like iTunes (iMix), Rhapsody (Playlist Central), MusicStrands, Mercora, etc. etc. I want to be skeptical of the potential since making playlists is work. However, there's enough evidence that enough people are motivated to do this that it could take off, if it hasn't already and I just don't know it.
The tricky bit is how to write transmittable, but rights safe, playlists. If a playlist points to licensed music, and I send the playlist to someone who doesn't have the same license, what happens? I was perusing the XSPF format and this appears to me to be fraught with a few technical issues: canonically identifying tracks, resolving how to retrieve them, yadda yadda.
But the DJ in me has a fascination with the potential for playlists as another social media.