Shelley Powers has a thoughtful post on tagging, focusing on the recent Technorati implementation of tags, but going far beyond in her thoughts about tags and structured data. This is a rich, complex discussion with lots of quotes and citations, illustrated with some kick-ass photos.
Su Chaw and others have equally wise comments, like this one:
"I can't help feeling that we're really only at the very beginning of the creation of meaningful tagsonomies and tagsonomical tools. Technorati's implementation of tags is one step on a long road - until we can sort by what Technorati calls 'authority' (but which is really a sort of popularity), pull the search results in to our aggregators by RSS, search using Boolean operands on multiple tags and do all sorts of complicated bespoke filtering, tags will remain a bit of a kludge."
This is a fairly technical discussion, but if you're interested in tagging, it's a great read.