Hey, look at this — Amazon is adding user tagging to product data. .
Rashmi Sinha has written an article called A Cognitive Analysis of Tagging. This is as much a note to me to read as it is you, as I only started reading it this morning and got part way through. I found this one on the DrunkandRetired web site. Benjamin Booth has written a response called The Present Failure of Tagging as well.