It tends to simply be a matter of preference and the languages/technologies that you work with.
However, in general, designers tend to favor the mac platform more often than not (even though the Adobe suite is available on Windows, so there's no hard reason why designers have to use a mac. So again, it comes down to preference).
For developers, it's a mix of OS X, Windows, or Linux.
There is also the school of thought that your development machines should match your sandbox/production machine as closely as possible, so that is sometimes a determining factor.
Personally, I develop on Windows, since I work with both PHP and C# Windows applications. So for me, the Windows platform makes sense as a development platform.