I just published an article on TidBITS on this very issue.
Basically, I don’t think the average Mac user needs it yet. AV comes at a performance cost that isn’t justified by the risks it addresses. It isn’t that Macs are more secure than Windows- it’s that they aren’t as big a target yet, and I’m not convinced that desktop antivirus will help much once Mac malware really starts proliferating.
If you are a lone Mac in a Windows environment you might need to install it to protect your Windows brethren (don’t be the vector that infects them – sending viruses you don’t even notice is not nice), and if you go to a lot of risky places you should consider it.
For the record, I don’t use AV on Mac or Vista, but I do use it on XP.
And if Apple is smart, they can finish off the Leopard security features and harden the platform enough that it won’t be as easy a target even as market share rises.
(I was amused reading the Slashdot comments, which I usually ignore. I don’t mind the criticism, but at least read the fricking article, guys).