One of the cases I've been working on recently is for an issue introduced by Microsoft .NET Framework 3.5 Service Pack 1. The fix has now been finalised and released on the MSDN Code Gallery.
When you get this error the symptoms are like this:
You didn't have the problem before applying the service pack (which you will have had installed if you installed Visual Studio 2008 Service Pack 1).
When you try to view a page in the designer you may see something like this: