I know most everyone has already seen this, but in case you haven't, I can't resist posting a link to Marcus Ranum's "The Six Dumbest Ideas in Computer Security". It is worth reading for his writing style alone.

An introduction from his article:
"Let me introduce you to the six dumbest ideas in computer security. What are they? They're the anti-good ideas. They're the braindamage that makes your $100,000 ASIC-based turbo-stateful packet-mulching firewall transparent to hackers."