Mac viruses have been around for a long long time. There aren't many of them, but there are always a few around. This one is nothing new. Macs are not BECOMING vulnerable, they've always been vulnerable. The difference between Macs and Windows is that on Macs you have to consciously do something stupid to get a virus. On Windows you don't - they can come through normal use.
That said, Mac users will need antivirus software when there's a virus that doesn't come from a direct user action like downloading infected software off the Internet. Until then the only Mac users who need antivirus software are ones that want do download applications from unknown sources. Ergo, advocation of Mac antivirus software implies that you're either protecting the person while they do something 1) illegal or 2) stupid. The latest version of OS X (Leopard) even warns you when you launch an application you've downloaded off the Internet. So you really have to ask for it to get a Mac virus.
One other thing - Mac viruses are hard to spread because they require stupidity on the part of the recipient and intent on the part of the distributor... Windows viruses often come passively in the background and so they can spread very quickly.
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