incrediBILL - 6:00 am on Mar 25, 2012 (gmt 0)
The theory is to DENY ALL, and than make exceptions for the visitors you choose.
Then punch a hole in the 'firewall' for browsers and allowed bots only. Cell phones are the only thing that's kind of tricky, and using browsecap.ini can help with that.
Everything else bounces off the website like it was a grade A circus trampoline.