For those puzzling over tags and tagging there is a useful entry on The Search Guy's WebLog:

Here's an interesting fact upon which I'll base the rest of my argument: people are horribly inconsistent when assigning keywords to documents. If you give two people the same document and ask them to assign a set of keywords to describe it, then the sets of keywords that they assign will agree only about 20% of the time. This was one of the problems that lead to the development of full text indexing systems. If we couldn't choose a few keywords from a document, we would use every word in the document as a keyword! [The Search Guy's Weblog : Weblog]