Uhh, guys? There's no reason that somebody couldn't package up a piece of malware on the Mac and offer it up for download.
Nobody ever
has, but it's not impossible.
Viruses and worms are difficult to get on the Mac, because they'd have to find some way to install themselves and spread. But there's absolutely no reason why someone couldn't write spyware, bundle it with a screensaver, and put it up for download. People would download it and enter an admin password, and it would gain the freedom to do whatever it wanted to the Mac. It's called a trojan horse, and it actually happened once. (A fake "Microsoft Office:Mac Test Drive" on P2P networks that was really a script that erased files.)
It's not hard to avoid, though. Before downloading any apps, go through
http://www.versiontracker.com/ or
http://www.macupdate.com/
and A) make sure the app is legit, and B)use the links to get to a "reputable" download site.
Immunity from viruses does not mean immunity from evil.