While this guy says that the Mac OSX is more secure than Windows, he also has some other conclusions in his little write up. First is the market share, with Windows still dominating the market. Next is that there are a lot more ways to infect an OS instead of hitting vulnerabilities in the OS itself: applications. Basically, if you aren’t aiming for something, you’re not going to hit it.
So while the Mac OS X has more vulnerabilities, it still remains a safer bet than Windows because it is not the direct target of attack.