"Hi, I'm a young, hip, modern woman looking for a laptop for under $100. I'd like to have a Mac but I'm just not special enough for them. Hey look, I found this one on the bottom shelf at my local Walmart, the manual is in Chinese, but it's exactly what I'm looking for. Congratulations! It's a PC!"
The reason they aren't any viruses for Macs is the hax0rs all own Macs and actually like them.
That said it wouldn't take much to make a crippling virus targetting OS X - especially when they are the FIRST systems routinely cracked during the well publicized hacking contests.