And Google Analytics does operating system statistics, too:
In the noise are FreeBSD, Solaris, NetBSD, Wii, and Playstation 3, in descending order, each with a tenth of a percent or less. Two surprises, here. First, people actually use Solaris? As a desktop? And, second, folks browse the web with a Wii?
These results are obviously not representative of the overall web, but it is less skewed toward Linux than one might expect.
Browser-as-a-function-of-OS is interesting. Half of the Windows users sport Firefox over IE. Slightly more than half the Mac Heads opt for Firefox over Safari, which begs the question why? Nearly all Linux users go with Firefox.
In addition to the aforeposted bias, these statistics exhibit another effect: People browse the web during the day, at work, where they might be forced to use Windows. Analyzing operating system usage as a function of day and time could prove fruitful, but it would introduce another bias toward enthusiasts. Ultimately, however, what matters is what people use and not why they use it.