No experience with Ruby, but the only people I ever see / hear talking about how superior Macs are for programming are people that do front-end design for websites or use Photoshop. Amusingly, though, none of them have even tried to use anything else for the tasks because they were just told by their moronic Mac fanboy teachers that Macs were somehow better for these tasks, and they've just stuck with what they've learned to use. My guess is that some people don't connect the dots between looking at something on a $50 refurbished Dell monitor at their business vs looking at it on the $500-$1000 monitor that came with a Mac.
Like others have said, the OS makes little to no difference whatsoever. There's nothing special about a Mac PC, Windows PC, Linux PC, or anything else that makes their x86 commands any different than the other. Maybe at some point in history it mattered, perhaps Photoshop used to have a lot of bugs on the Windows version or something, but I have no idea.
All I can say is that now, there are little to no limitations with Mac or Windows (don't know about Linux since I don't use it) beyond just making sure whatever software/compiler you use runs; platform-specific bugs / feature differences are pretty few and far between for most things. But, hey, maybe one piece of software does happen to have that feature or bug on a certain platform and not others and it's something you really need/want/much better supported; go with that platform if that is a scenario you find yourself in.
As for this, there is Bootcamp to boot Windows on a mac for the windows solutions, and Mono for Linux (though not 100% support of .NET with it). PHP development is perfectly fine on Windows, as is Java (possibly Python/Ruby; haven't used either, but I've got a copy of JetBrains IDE for both so you could probably write the code for them without a problem, but they might be easier to set up / maintain on Linux). Serious graphic design only being viable on a Mac is a gross misconception.
Just from my personal experience, at least.