OSX is harder to attack (i have an iMac, BTW), but not impossible. You are wrong that there are zero instances of worms/malware/spambots. In fact there were recently numbers released that macs made up 16% of the infected computers in the Jnanabot network. Apple has been recommending anti-virus software for 3 years now. Your attitude is going to make it worse for mac owners because you are discouraging caution which is clearly warranted.
http://www.symantec.com/connect/blogs/exploiting-jnanabot-fun-and-profit
OSX is harder to attack (i have an iMac, BTW), but not impossible. You are wrong that there are zero instances of worms/malware/spambots. In fact there were recently numbers released that macs made up 16% of the infected computers in the Jnanabot network. Threat Assessment
Wild
Wild Level: Low
Number of Infections: 0 - 49
Number of Sites: 0 - 2
Geographical Distribution: Low
Threat Containment: Easy
Removal: Easy
16% of 0-49 infections = a maximum of eight Macs. My hypothesis is that those infected Macs were running Windows in boot camp. As described in the technical details on the Symantec site, Jnanabot only affects Windows; it infects a system by writing to the registry and downloading and installing several .exe files. The numbers in that pie chart are "from artifacts of the file system."