When trying to add a control to the toolbar you get this error: There was an error loading types from assembly 'C:\filename.dll'. Could not load file or assembly.

The solution is to put filename.dll in C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\PublicAssemblies and then add it to the toolbar by browsing to it. This seems to be a permissions issue but by using using Filemon I was able to determine that VS 2008 at least had permission to access this directory.