A recent article on Forbes talks about a Lieutenant Colonel of the Army purchasing Apple Macintosh computers to decrease their risk of exploitation. Primarily in response to the recent security breach of the Pentagon back in June as well as a few other incidents. It is widely discussed that Macintosh computers are more secure than Windows & Linux based computers because fewer vulnerabilities exist for the Mac platform.
What I never hear talked about in these discussions is the alarming fact that Macintosh had five and a half more vulnerabilities per month on average than Windows throughout the year 2007. You can see the details and the numbers in a recent ZDNet article. It is quite common to see Macintosh users without any active Malware (Anti-Virus, Worm, Trojan, Spyware) protection.
Back in April 3Com held a short lived contest that resulted in compromising a fully patched Macintosh laptop for a prize of $10,000 and the MacBook.