Hi,
I compared MediaWiki's category system and its usage in Wikipedia with
traditional classfication and collaborative tagging. In my impression of
discussion about categories people confuse a lot so maybe my paper can
help a bit (or confuse you even more - it's scientific ;-)
http://arxiv.org/abs/cs/0604036
http://wm.sieheauch.de/?p=35
Greetings,
Jakob