I'm a strong advocate for OS X as a desktop OS, but it's worth pointing out that OS X was really late to the party in having a half-decent ASLR implementation, and has earned much of its reputation for security by being a smaller target in terms of market share. I'm not saying Windows 7 is more secure if you control for the market share effect, but I bet it's closer than most people think, and most of Apple's gains came from the most recent 10.7 Lion release, while the claims about being more secure were made when OS X was still vulnerable to many classes of attacks that other OSes solved many years prior. (Of course I'd rather get pwned and restore from Time Machine several times a day than use Windows or any Linux distro as my main desktop machine.)

Given how long it took Apple to issue a patch for this very prominent exploit, I think they'd be in trouble if OS X were suddenly on 80-90% of all desktops tomorrow.