If you use either the “Black Ice” or “Real Secure” firewall, your computer is at risk. The worm doesn’t use email; from the unclear description in the article, it uses a buffer-overflow attack to affect the firewall software directly.
Once infected, the firewall software randomly overwrites your hard drive and presumably does network probing to find other computers to infect.
Nevertheless, if you’re concerned about security or your Mac’s on a network behind one of these P-O-S “security” applications, you can turn on the Firewall feature in Mac OS X (System Preferences -> Sharing -> Firewall). To date, I haven’t seen a single hack or vulnerability for OS X’ integrated firewall.
The greatest threat to Mac users is Apple going bust. People have been predicting it forever. If you believe the whispers, even iPod isn’t going to save 'em. Think it’ll ever happen?