Now that the Blaster worm is spreading it is once again time to think about biologically inspired anti-virus defences. The absolutely coolest way to combat a virus like Blaster is to spread the security update that protets against blaster by USING the security hole the patch is to fix. i.e. spreading a benign worm.
If Microsoft did this there would be a public outcry of course - the two problems involved is the possible invasion of privacy (pirated windows versions would NOT have the automatic Windows Update enabled and would therefore be particularly susceptible to the worm) and of course the difficulty in distinguishing benign viruses from non-benign ones. Viruses that pose as security updates are already out there. And last but not least a benign virus would be a colossal liability for the spreader. People might not agree on what constitutes benign after all.
From what I understand this is a problem with spam also: Spamblocking software by its very nature may block what some consider ligitimate email which could both cause legal problems for the blocker and of course raises freedom of speech issues. With paper mail I think it is the law (at least in Denmark) that a sender has the right to insist on delivery, regardless of the opinion of the intended recipient.
This is only now being limited wrt mass email.