Mads Laumann got this error in a Silverlight 4 project and after he googled it for some time without any luck, he decided to write a quick blog post just to help others which might have the same problem.
Today I got this error “XAML Namespace http://schemas.microsoft.com/expression/blend/2008 is not resolved.” in a Silverlight 4 project. I happened right after I added some VisualStates in Blend 4. I could remove the VisualState XAML and everything would then build again fine. As meantioned I googled the error but couldn’t find a solution.