Design View for Xaml on Visual Studio Professional RC 2012 used to work for me. But suddenly stopped for no reason I can figure.
Check the following:
Tools -> OptionsText Editor -> XAML -> MiscellaneousOr perhaps the default program for opening .xaml files has changed:
.xaml file in Solution Explorer Open With...XAML UI DesignerSet as Default button OKEDIT
This does appear to be a known bug with an earlier version of VS11.
I had the same issue in Vs 2012 express edition. Solved by running vs2012 command prompt and executing the code
WDExpress/ResetSettings
If anybody is interested. I had the failure: Could not open XAML and config files in Visual Studio 2012. Now I detected the cause: the Solutions suo files were corrupted after a System Crash. So I (was not sure) rebuilt the sln file in a different Folder and copied the sln and suo files (they are hidden!) to the old Folder. That definitely brought me back the lost functionality. Maybe it is only necessary to delete the old suo files. Juergen
In Visual Studio 2013 I had a similar issue - I change my build action & from BundleResource to AndroidResource and I was able to get it working.

Suggestion 4 in this link worked for me:
devenv /resetuserdata in Command Prompt.
So at least in my case, re-installing and then the above command cleared all issues. It is possible that resetting user data could have made it unnecessary to re-install but I can't know that now.
I still can't say what caused this but it seems a pretty long tail issue that occurs sporadically and looks to have precursors in older versions.