SYMPTONS
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

 or

Package Load Failure

CAUSE
Visual Studio failed to load the dlls required by Silverlight Tools.

RESOLUTION
To resolve this issue, from a command prompt, run "devenv /resetskippkgs"
(i.e. "Folder Path\Common7\IDE\devenv.exe" /resetskippkgs)