Let's not read this out of context. ASLR and DEP make buffer overflows more difficult to execute. However, as indicated by so many exploits in Windows Vista and several already in Windows 7 indicate, these do not "prevent" them.
When looking at which operating system is "more secure" we must look at which allows unwanted code execution in general; not only buffer overflows. We must also look at which operating system discloses more information, which allows easier cross-account local access, etc etc. In all areas in question, either due to more effort or less security measures, Windows is far an away less secure than any *nix-based system, including OSX.
I work in OCIS on a college campus. Our biggest problem has been with malware finding new ways to get its self installed through IE, Firefox, and/or Windows holes. Seems like nothing short of white-listing can effectively prevent this kind of stuff (eg. AntiVirus 2010). I use mac and linux boxes at home except for the one Windows computer used for a gaming system for the family. I am yet to see anything ever get inadvertently installed or executed on my mac or *nix systems. EVER.
Therefore, say what you will, but OSX and *NIX systems have consistently been more secure in the real world. When targeted specifically by an experienced security enthusiast, I would imagine that "the most secure system" varies from one day to the next, but I trust my Mac 10 to 1 over any Windows box.
The next time you think about the topic ask yourself this. Would I feel good about letting a random 10 year-old use my Windows computer without monitoring him? I wouldn't. My Mac on the other hand would be just fine assuming he didn't drop it.