Quoth Mark Pilgrim:

At its heart, anti-virus software is little more than a glorified blacklist. It maintains an internal list of evil applications and their unique byte signatures, and if it sees one on your system, kills it for you. Sure, anti-virus vendors will dazzle you with their ad copy, their heuristic this and statistical that; they’ll tell you (with a straight face, even) that their software is far more than a simple blacklist. It’s a blacklist with lipstick. It’s the prettiest, shiniest, most kissable blacklist you’ve ever seen!

Relying on a blacklist model for security is tantamount to admitting failure before you’ve even started.