Matz, Ruby's author only got his first mac last year and I believe he still does most of his work on Linux.
However, most of the US developers, starting with DHH were on Apple when Rails started to become popular.
Until recently, Ruby on Windows was a very painful thing, the problem isn't fully resolved but it's better now.
MacOSX is built on bsd, offering the low level tools + a well designed, nice to use windowing system.
Of course TextMate also helped things up.
Finally, Apple decided to embrace Ruby, bundled it with the OS then shipped Rails with the OS.
Ruby and Rails are used by Apple itself and they invested in MacRuby, their open source Ruby implementation that runs on top of the Objective-C runtime.
So, to sum up, the Ruby community in the US openly favorited Apple and Apple returned the favor. If you have the opportunity to come to a Rails conf, you will more than likely see a great majority of Apple laptops.