Personally I don't think there is any real advantage that a Mac (or specifically developing in OSX) would provide for Rails development.
If I were to be more mean-spirited, I would say that people who use Macs are the kind that like to jump on the "new and shiny" bandwagon and Rails is definitely one of the more popular and "cool" things out there.
Note: I don't really believe that, but people love to hate on Mac users.
