Not to spoil the party, but THERE ARE MAC VIRUSES... The fact though is that most of them have minimum impact and may affect only independent programs as few if any resources are actually directly integrated withing the OS. By comparison, in MS software most programs work through the operating system and thus a virus can affect the entire computer.
See
http://antivirus.about.com/od/macintoshresource/ for some examples of viruses...
There are very few if any viruses currently circulating, and smart internet behavior will probably eliminate the risk of getting them. Still, it is very ignorant to just think MAC cannot get a virus. I love OSX and I'm a recent switcher, so coming from Windows, that was a big concern of mine. You are right, it does not seem to be a problem for Macs but in the next decade with my predicted rise of apple use, it may as well be a problem. As long as there's a computer some brilliant hackers out there will be able to mess it up. Right now, it's not fun to mess a Mac cause you will not get much impact with only about 5-10% of the market being apple. However, the more Apple makes commercials of "Oh, we don't get viruses" - some lonely person out there is gonna set to prove them wrong... Can you imagine the fame associated with that...
Also, since most users are Windows, a mac virus has a small chance of spreading too... As files have different protocols and programming, so a mac virus would get stuck on a Windows PC... haha