Matt Asay discusses the seemingly incongruous rise of the proprietary Mac in open source community. He makes a lot of good points, including the ability to quickly evoke terminal and run Unix apps while at the same time maintaining an attractive desktop interface, but I think this statement sums up why many have switched:

At a certain point, I just want something that works well.

Which isn't to say that Linux or Windows can't work well for some people, just that for those who switch, the Mac works better. Many of the code monkeys I know who've switched have done so because they're tired of fighting with computers, code and related crap all day; they don't want to have to do the same on their own computer.

It just works. And that's worth a lot to geeks who'd rather spend their time on meaningful projects then figuring out why the damn printer won't print.

And the flexibility of the Mac as a platform, particularly a web development platform, can't be understated. You can get a Macified version of LAMP (Linux, Apache, MySQL, PHP and/or Perl, with Mac OS X substituted for Linux) up and running in no time, and can easily add other languages and frameworks, like Ruby on Rails. Of course, you can do that on Linux and Windows as well ... but for my two cents, it's a hell of a lot easier on a Mac.