I have been using this script for a couple of weeks now and it really seems to help identify mischeivious people as well as spiders that are looking for missing files (robots.txt). I put a meta refresh in the header to redirect a human user to the home page.
Everyday it seems I get a visit from some new spider that I would probably miss otherwise. Here's the link
Apache Guardian [xav.com]
Does anybody use a similar, perhaps better system?