Rashmi Sinha has another nice piece on tagging, A social analysis of tagging:
I strongly believe that all good social systems need to serve the individual motive. Tagging works because it strikes a balance between the individual and social. It serves the individual motive of remembering, and forms a ad-hoc social groups around it. If you are designing a tagging system you need to understand how it serves the individual and what sort of social formations it supports.
I like the way Rashmi talks about the blurry line between the individual act of tagging and the social use of those tags. People tag to remember something. But they also tag "for" the larger world, either to say "hey, I found something of interest" or "hey, we like the same thing." And the result of lots of people doing lots of tagging is that one can surf through tags or through taggers to find other interesting materials (and possibly spend a lot of time). You might also discover a community -- or create one through the use of unique tags or the adoption of tags for a community.
The ideas that Rashmi describes have been discussed pretty heavily in the CPSquare conference, with experts and neophytes interacting over a variety of "web2.0" topics as they relate to communities.