According to Symantec it is. In their 11th internet security report, part of which tracked vulnerabilities and fix times for operating systems in the last half of 2006 they found the following:

Microsoft's Windows came in first place with 39 vulnerabilities discovered and Microsoft took on average 22 days to fix them.

Red Hat Linux came in second place with 208 vulnerabilities and an average fix time of 58 days.

Apple's Mac OS X ranked 3rd place with 43 vulnerabilities and an average fix time of 66 days.

Hewlett Packard's HP-UX had 98 vulnerabilities and took them 101 days to release a fix.

Sun's Solaris came in last place with 63 vulnerabilities with a fix time of 122 days.

Looks pretty much the same as the first half of the year. Microsoft's security investment is paying off. 2006 was a solid year for Microsoft on the security front. With how 2007 has started its going to get even better.