So it has been a while since I needed to write a legacy ASPX application.  I updated my Telerik controls to the latest version which changed the manner in which the design time control code is referenced.  As it turns out Telerik has gone full circle with their controls as far as how the assemblies are distributed.  They used to put each control in its own assembly then tidied it up a bit by placing everything into one assembly.  With the latest revision they separated the design time code from the redistributable. 

As it turns out Visual Studio 2008 RTM had a problem where design time controls would not load if the DTC was GAC’ed.  SP1 fixed this issue and all is OK in the world of Design Time Control references from the GAC.

This is a key piece of functionality.  As a developer that has needed to build controls with designers so that developers could consume my work… I understand how frustrating this can be.  A software rev (Visual Studio) breaking long working functionality.

We update your Visual Studio if you are using legacy ASPX pages/controls and you are having Designer Rendering issues. 

Download details- Visual Studio 2008 Service Pack 1