Hi
it's not really accessible. Don't bother.

The closest thing I'm aware of to cross-platform is HTML5 and Javascript, and only somewhat. And those seem to require any resources be loaded from the internet (every time; browsers have forbidden local files). And the audio APIs are a mess (the best bet seems to be howl.js).

Hi Yukio,

C# is another option for cross platform development, and in some ways the closest to Unity3D since they both use mono to work on operating systems other than windows.
I have made C# apps that run on Windows and Linux because those are the operating systems I have easy access to, but Mac also supports mono.
If you want C# to run on iOS or Android you have to purchase a special Xamarin license to do that.
Unity3D also charges you to deploy to iOS or Android.
If you use native dlls then you'll have to ensure you only use those that can compile on all your target platforms.
There is a free framework called MonoGame that gives you a bunch of cross platform game related libraries.
http://www.monogame.net/
I'm pretty sure Xamarin Studio is not accessible, but Visual Studio is reasonably accessible and you could probably use the command line Xamarin tools to compile for the smart phone platforms.
Visual Studio is free, and it is free to compile for windows, linux, and mac.
It looks like compiling for iOS requires a Mac.

Unity3D is basically C# and mono, but they've added a bunch of helpful game libraries and tools, and gone to the effort of making sure they all run cross platform.
MonoGame is similar, but not a commercial product so doesn't have the quality or broad set of tools that Unity3D has.
And as mentioned, you purchase the ability to compile for iOS and Android directly through Xamarin, instead of through Unity3D who must have their own deal with Xamarin.

All that said, this is just one option.
I haven't compiled for the smart phones myself, but I have used C# and Visual Studio for a very long time, so can help with those if you went this route.
And you would still want to verify that Xamarin has command line compilers.
Good luck with whatever technology you choose.

Pythons a fairly good cross platform language, though you'll have to find and tinker with the required supporting packages. Pyglet, PyAL, Pygame, and other API's are good for windows, mac, and linux development, if your going for mobile you should have a look at the Kivy API. I don't know its level of accessibility, but another choice might be HaXe, which is also cross platform and allows you to compile code into other native languages.

Thanks for the correction Craig.
That announcement came out a week and a half after my post and was certainly welcome news.
Also good to hear that Xamarin is now integrated into Visual Studio since last time I tried Xamarin Studio on windows NVDA just reported unknown for the entire window.

Cool, I'd love to hear how it goes if you get some time to try it.
My brother uses a framework called MonoGame to make his games work on Mac, Linux, Windows, iOS, and Android.
But I know that Xamarin also has frameworks like monoDroid and monoTouch for wrapping the native android and iOS GUI frameworks and other libraries.
Which you use sort of depends on wehther you want to use preexisting controls or custom render everything yourself as a game usually does.
I haven't tried any of it myself though, so will be most interested to hear about your experience from an accessibility standpoint.