I can say that the majority of skilled developers I know use Macs or Linux, and I know a pretty good number of developers (skilled and not-as-skilled).

If you yourself use OS X or Linux, you are much more likely to know primarily people who are similar in this respect (for the same reason that I as a .NET guy don't know many Rails developers), and in any case jdietrich's statement lacked the qualifier.

Totally, I mean you pretty much have to be on Windows to be a good .NET developer (unless you're big on Mono or whatever).

It sounds like the OP is interested in working with "latest and greatest" web technologies, though (no offense), and many of them are developed on Macs or Linux and therefore the best development environment for them is on a Mac or Linux machine.

I think it might just be a culture fit thing, as some people gravitate to .NET type stuff and some people gravitate to node.js or Rails, and just as it's a pain in the ass to get .NET stuff working on a Mac, it's often a pain in the ass to get things like Rails or node.js working on Windows.