The simplest answer is that you can easily create RoRs on any platform.
Personally, I started developing on Windows, and then switched to VirtualBox on windows running Ubuntu. I now have a MacBook Pro and it must say that this is the happiest environment!
Windows - It's wise to just install rails, but it seemed much slower than in other environments. The disadvantage of a simple console with color backlighting was annoying.
Ubuntu - faster than under windows (even with virtualization!). If you don't know Linux, it's a lot less fun, a steep learning curve for the command line and lots of dependencies to solve
Mac - The best of both worlds, a nice interface for everyday work, awesome command line support, Ruby and RoR work very fast on the MacBook Pro.
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