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

I do include the first 3 months of Windows Vista as well, which had 1 fix 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, such as RHEL5, starting next month.