Hello!
Just yesterday i have had the same problem, but in vs2003. The answer was simple (in that case) -- errors in c# text (i edited the text manually) made it impossible for designer to treat the objects defined in the text as consistent controls.
A simple check is to compile the form. If there are errors in sensible parts of the program (component declarations and deserialization), they must be fixed first.
HTH,
draiko