InternalsVisibleTo !
So, we intend to introduce a new feature in next release of .NET -- "Friend Assemblies", that does precisely what you're suggesting. It will use an assembly-level custom attribute called "InternalsVisibleTo", that grants access to a named "friend" assembly (Omer.B in this example) -- for all "internal" types.
- Jim Hogg [from here]
Exactly what I was looking for earlier today ! Until then I guess we have to rely on the StrongNameIdentityPermissionAttribute to limit access to our assemblies.