WPF designer cannot render most forms

I have written a few projects in WPF now. What I don't understand is, why is the designer so fragile ? It seems as soon as I create any sort of reusable control, and use it on a form, even though the control exists in the same code base as the form, the form will no longer be able to render properly, it will report errors, which are invariably 'can't create an instance of MyCoolControl'. Of course, at run time it works fine, and the code for the control is right there, so I don't really understand what the issue is. I have talked to other people who have THEIR controls inside seperate dlls, which is apparently the recommended way to resolve this 'issue'. They still have the same error, and the way they have solved it, is by putting those dlls in the GAC.

Wait a minute. If I want to create a control inside my WPF app ( which is surely the OO way to do things, create it once and reuse it ), then I should create it in a seperate dll, and THEN put that dll in the GAC, so that the designer will work properly ? Am I the only person who thinks that is insane ?

Of course, I've not seen VS 2010, there is some possibility that this has been fixed. But, for now, it's a constant annoyance.