While writing a new article, I have noticed that these two articles required some updating.
Since this article was written, a new GAC location has appeared with .NET Framework 4.0 (in C:\Windows\Microsoft.NET\assembly), that furthermore doesn’t have a shell extension:
HOWTO: Reference a Visual Studio assembly in the GAC from an add-in
And I have added C# sample code to this other one:
HOWTO: Get a Visual Studio service from an add-in