I just posted my January 2007 - Operating System Vulnerability Scorecard over on CSOOnline, which includes charts comparing the vulnerabilities in Windows, Red Hat Linux, Ubuntu, Sun, and Mac OS X, broken down by server and workstation.

I do include the first 2 months of Windows Vista as well, which had no vulns fixed in that time period.  As a teaser, here is one of the 4 charts from the post:

I plan to update this monthly throughout the year, and will include newer products when they are released.