It always says "The solution "foo.sln" is of an unsupported Format and may not open correctly. Do you want to open the solution anyway?" If I click yes, it Shows the solution tree, but when I open a xaml file, it only Displays its xaml text, not the Rendering of the UI.
According to this http://blendinsider.com/technical/available-now-blend-for-visual-studio-2012-2012-08-15/ Blend 4 is the way it's meant to work. So something is wrong I guess.
I have older Projects created with vs2010 and Blend 4. I can still open and work in them in Blend 4 while working on them in vs2012. Only new Projects created with vs2012 seem to have this Problem for me.