Initially developed for the Windows Phone, it now works in iPads, Safari, Chrome and even IE 10! This means you can use the Surface or other Windows RT devices to build your apps. Well not quite, but close. To build a Windows 8 Store app or Phone, either 8 or 7 you do need Visual 2012 running on Windows 8. If you are a student that is pretty easy to do, but for non-students you will need the Windows 8 and then use the completely free Visual Studio 2012 Express for Windows 8 to build Windows Store Apps
Check it out at http://www.touchdevelop.com/app this is just the start. Cool stuff.