At the recent CanSecWest Applied Security Conference that was held in Canada, three computers running Mac OS X, Vista and respectively were offered for hacking. None of the three were compromised remotely, i.e., via network access, on the first day.
The second day, with rules relaxed a bit by allowing hackers access to all applications that come bundled with the OS, the Mac OS X fell via a Safari web-browser vulnerability. The hacker setup a malicious web site and was able to gain access to the computer after Safari was pointed to that site. Lesson here, don't go to sites that you are not familiar with.
The third day saw Vista fall after the contest allowed third party applications to be probed for vulnerabilities. Adobe Flash was found to be vulnerable, making Vista fall.
Of all three, only Ubuntu Linux was left standing! Talk about security! Bwahaha.