For the most part rails applications are developed on OSX and deployed on to linux. So those are the two platforms where all the libraries tend to just work. If you go off of that to, what for rails are, non-standard OS's like Solaris or Windows, things might not work as expected.
I would say there are good arguments for choosing a Mac over Windows in terms of ruby on rails development. But there is no real advantage of using a OSX vs Linux. Apple hardware works well, it's OS works well, it's enough like unix that things mostly work. Installing libraries work, you've got the commandline always, etc...
Lots of people point to textmate, but the reality is among many in the rails cabal the move has been to abandoning textmate and moving to either emacs or vim.
Rails development on windows in a painful hackish task. And i've never even heard of somebody attempting to deploy production rails using windows. It's not that windows couldn't be made to be a first class ruby / rails OS, it's just that there are not enough developers using the platform to keep pace.
