It's always been a huge pain for me that the only way to reliably separate components in C# was to put them into separate projects.
I'm sure everyone knows the feeling, when you don't know what accessibility modifier to give to the entity: "public" or "internal". That's relative to the scope! And "current assembly" is not the scope we are usually thinking about.
There are different approaches that could solve the problem. We could introduce notion of "component" into .NET languages, but it's obviously a very expensive solution.
I myself prefer another approach. Let's continue improving architecture tools that appeared in VS2010. What if layer diagram would be able to verify not only that one component (or "layer") has no dependencies on another, but also that component A has only public interface dependency on component B?

It's always been a huge pain for me that the only way to reliably separate components from touching each other's internal stuff was to put them into separate assemblies.
I suggest you to improve layer diagram to be able to verify not only that one component (or "layer") has no dependencies on another, but also that component A has only public interface dependency on component B.