I'm as big a fan of tagging as anyone, but the rampage to determine if tagging can or can't generate useful emergent taxonomies befuddles me. At least in the del.icio.us incarnation, tagging is supposed to be highly personal. You're just marking your stuff. Who cares if it confuses the heck out of other folks.
Peter Merholz observes that social effects are overriding any metadata imperative. Tags are being used for signalling not taxonomizing. Instead of marking to help people categorize stuff, you mark to announce to the world that some piece of media is now available for a particular audience. Turning every tag into a post-and-poll stream like del.icio.us and Flickr, or tying into an engine with search feeds, like Technorati, makes it easy to track the signals.
The key difference is that with signaling, people only think in the moment. There's no sense of posterity, which probably leads to impoverishd metadata. I mean who wants to think about the right way to tag that party picture?!