Nobody cares about OS X (very small market share), that's why nobody writes virus for it
As a matter of fact is not so hard to write one.
1) Find those buffer overflows in image handling libraries before Apple
http://secunia.com/advisories/36701/
2) Find a privilege escalation bug before Apple
http://www.macworld.com/article/134165/2008/06/ardagent.html
3) Generate images containing the virus (wallpapers, banners, etc) and upload them to as many sites, torrents, etc. as you can
4) The virus should be very quiet and infect newly created images and one of the default system processes
5) Profit!
As a matter of fact is not so hard to write one.
1) Find those buffer overflows in image handling libraries before Apple
http://secunia.com/advisories/36701/
2) Find a privilege escalation bug before Apple
http://www.macworld.com/article/134165/2008/06/ardagent.html
3) Generate images containing the virus (wallpapers, banners, etc) and upload them to as many sites, torrents, etc. as you can
4) The virus should be very quiet and infect newly created images and one of the default system processes
5) Profit!