iTunes 4.5 is out, with support for lossless imports (via Quicktime 6.5.1), WMA import on Windows, music videos…and shared playlists. You can click any playlist in iTunes and publish it to the iTunes Music Store. You can also click any song title, album title, or artist and jump to the appropriate content in the iTMS.
I don’t have any good playlists on my work computer (mostly random shuffle things) but I have a ton at home, and will publish them in the store tonight. I’m curious to see how the feature (which appears to be called iMix) will handle tracks that aren’t for sale; probably it will just omit them.
Update: Behold, my first iMix playlist, a version of this mix. The difference is all the songs I listed as “missing” (i.e. not available in the iTMS). Vote for it, won’t you? I also noted looking at some other mixes that user submitted mix names get passed through the same profanity filter that song titles do, but that it appears to be possible to elude the filter by choosing your slang appropriately.