It's pretty much 100% confirmed now. Providing MS implement it in a standards-compliant way, all existing Windows 8 apps should automatically switch to using the faster WebGL renderer, and allow use of shaders. I hear it's also coming to Windows Phone 8, although later in the year. So it's good news for anyone publishing to Windows 8 or Windows Phone 8.
For the web, this means WebGL is much more widely supported. IE still has a pretty big chunk of users on Windows, and if they all end up on IE11 then that's a lot more people who get the faster, shader-enabled WebGL renderer. So good news for web publishing too!