Quoted from TechWeb:

Tallies kept by the U.S. government's computer security group show that Linux and Unix operating systems faced nearly three times the number of vulnerabilities in 2005 than did Microsoft's often-maligned Windows.

In the US-CERT (United Stated Computer Emergency Readiness Team) year-end vulnerability summary, Linux/Unix accounted for a whopping 2,328 vulnerabilities, about 45 percent of the 5,198 total.

Windows, on the other hand, sported just 812 vulnerabilities during the year, said US-CERT, or 16 percent of the total.
My guess for this is that most of the Linux vulnerabilities are pretty small, whereas in Windows' case they are larger ones like the WMF Exploit - but I haven't really had time to look at CERT's end of year report.

Any observations are welcome

-jk