Virii are primarily written by script kiddies.
Script kiddies exploit the GAPING holes in Windows, and thankfully OS X lacks these gaping holes.
There are security problems with OS X (and with every OS), however they are usually beyond the scope of script kiddie talent.
Also, script kiddies will continue to target Windows exclusively until OS X virii make onto CNN. Marketshare means this is unlikely at the moment.
In the future, we may see more Trojans, and more elegant penetrations of OS X, likely perpetrated by disgruntled programmers, however, Virus propagation on UNIX is orders of magnitude more difficut than on Windows, so we'll be likely to see stealthy machine control hacks, or DOS attacks.
I could be wrong...there might be some really smart script kiddies using/hacking *NIX, but I really think teenage gamers are the predominant demographic writing virii, and they primarily use Windows.