[duke_sam]

#2 is partially handled by rdio (http://www.rdio.com/) you can create and share playlists and any number of people can listen to them. Not sure about collaborating on playlists but considering the heavy social aspect of the site I would be surprised if that feature never arrived.

[JED3]

This is true, but Rdio still lacks the synchronized playback to the followers of a list, unlike what we're used to while listening to a real 'Rdio'. I believe this is what makes paulgb's idea proposal unique and all the more attractive to me that something out there would allow me to "listen to the music at the same time" while collaborating on what gets played next.

Examples: http://hackermusic.com, http://tunez.sourceforge.net/

[alexophile]

Yeah, when you create a playlist on Rdio, you can choose collaboration options: anyone can edit; only followers can edit; nobody can edit. And, as far as I know, there's no limit to the number of concurrent listeners.