There’s a really retarded issue in the Blend 3 RC where it will not allow you edit a XAML file in design view. The problem is, Blend 3 RC looks at the project type when it loads it, if it’s not a project type that “supports” XAML (Like a class library) you will not be able to edit any of the XAML files in anything other than the text editor.
A quick dirty way to fix it if you have already created the project is to open the project file in notepad and under the <PropertyGroup> tag that contains the <ProjectGuid> tag, paste this line:
No spaces. Reload the project / solution in Blend 3 and you should be good.
Hope it helps some poor soul out there that has begun swearing that they will never ever use the latest version that Microsoft has released, just one version earlier.