Macs and Windows have one thing in common: HUMANS. Humans make the OS and applications, and humans use them. Since we are still not perfect, there will be issues. Also, Doug, why do you hate on Microsoft so much? When it comes to security Microsoft has a much better track record than Apple, Adobe, Google, Sun, etc.
-Stuart
Macs are, in my opinion, more secure than Windows. Unix or Unix-like operating systems start with a more restrictive set of permissions for the average user than Windows. However, "more secure" doesn't mean they are secure. Any OS, be it Windows, Unix, Linux or Mac, is more vulnerable if standard security practices are not followed on a regular basis.
-Chris
Being system agnostic in terms of support, but a Mac person in terms of personal purchases and preference, I take umbrage with the "decade behind" comment. I believe Macs are inherently more secure than Windows because of the Unix foundation, but simple popularity is the larger factor. With Apple's increasing popularity, it was only a matter of time until Mac users had to buy antivirus software. I don't think that day has arrived yet, but it may be as soon as some time this year.
-Corey
I would add that there is probably a sizeable population of Macs that are not very secure, not because of what Apple has or has not done, but because of the flawed notion that if you just buy the right product, you won't have to do anything because it is a superior product. Security happens by the practices you follow day to day, and the processes you put in place. If you don't put any in place, then you're destined to discover the flaws of your assumptions sooner or later.
-Dan