Looking at tag counts,
[linux] tagged questions outnumber
[windows] tagged questions just shy of 2:1. As for our answerers, looking at who of our top-10 have earned which tag-badges:
Windows: 5/10, +1 silver
Linux: 7/10, +4 silver
So this does show a noticeable preference for non-Windows here on SF. In my experience this is not unusual for sysadmin communities. It is notable that of our top-10, there are several of us who have tag badges in both Windows and Linux, which shows the sort of cross-platform familiarity you have to have to be in this field for any length of time.
Personally, I started as a NetWare admin, and got into Windows when our Windows guy left. That guy was a self proclaimed Solaris bigot who got informed by management that he was getting to manage some new WinNT boxen, which he did for about 9 months before getting fed up and going to work elsewhere. I was not displeased.
I leveraged this dual-stack to get my current job. Now that NetWare is a dead platform (even Novell says so these days!) I've been trying to lever Linux onto my resume to keep that 'bilingual' feature. Unfortunately for me, what I know about Linux is not web serving, high-availability or otherwise, which is where most of the 'Linux' skill-set seems to live (at least judging by SF questions).
At work these days I'm 95% Windows, 5% Linux. At home I'm all Linux, with an occasional pinch of Mac and a dash of Windows.