Personally I think 80% of the people that discuss "security" randomly on forums don't really understand what makes a machine "secure" and what doesn't (including me). Like so many other internet-y things they end up as regurgitations of information skimmed from articles and blogs here, there and every where. Sadly, these regurgitations have a way of becoming "fact" and "truth" when there's either minimal truth or none at all. If enough people on the internet say it's true, then it must be true!
There's an absolute TON of moving parts in the machine of "security". Being able to call an OS "secure" or "insecure" takes a very deep understanding of all the parts involved. Something that few folks really have. It's just to easy to play the armchair "expert" and take up the call and promote FUD.
Undoubtedly there are mind-boggling amounts of exploits in Mac OS X & Windows, some of them so obscure that only the truly gifted could find them. But find them they will. Write articles about they they will. Eventually someone will take the info and twist the facts to use for self-serving purposes.
Personally, maybe the claim that Vista is more secure than Mac OS X is true. Why? Because no one is running it. Therefore, security by obscurity plays in Vista's favor.
All I know is 90% of what all my colleagues are doing right now is dealing with Windows viruses. It'd been more or less quiet for quite some time and suddenly in the last 3 months..
Basically it boils down to this: OS X (including SL) is safer than Windows. It is not more secure than Windows. Safe != Secure.
Once Apple implements full ASLR with NX, it will be as secure as Win7 64-bit. Miller has said in another interview (which I can hunt down if you wish) that he suspects that once Apple flips the switch to full 64-bit, they will implement this, possibly in a later SL patch.