Basically I have quite a few artists in my collection who are fond of collaborating with other artists. It would be good if I could put all the artists in with a separator and the song in essence belong to each of those artists so I my Album "Rolling through this world" by "Bob Bozman" and "Jeff Lang" would appear under both "Bob Bozman" and under "Jeff Lang" as opposed to having to choose one of them or creating a new artist called "Jeff Lang and Bob Bozman".
Similarly I have a lot of music that doesn't neatly fit into one genre so I would like to be able to have"jazz,metal,70's" the genre as say "jazz,metal,70's" and have this interpreted as belonging to the genres "jazz", "metal" and "90's" as opposed to one genre of music called "jazz,metal,70's"

That's probably as clear as mud... If that could somehow fit in with the last.fm tagging system that would be even better

something like the last.fm tag system is being looked at - will be a while though, I'd think.

There's a thread somewhere else on here about it.

The artist thing is trickier - I do end up either creating a new artist for collaborations, or using (feat Artist2) after the track title. Both of these are acceptable solutions according to musicbrainz style guide.