When creating a new Silverlight project or opening an existing Silverlight project, you receive an error message:
Object reference not set to an instance of an object
Package Load Failure
Visual Studio failed to load the dlls required by Silverlight Tools.
To resolve this issue, from a command prompt, run "devenv /resetskippkgs"
(i.e. "Folder Path\Common7\IDE\devenv.exe" /resetskippkgs)