Does VS.NET Build Project propagate the compiled DLL up the dependency chain?

[spargap]

We just ran into surprising behaviour in VS.NET 2010 SP1. We're not sure if this is new or we never noticed it before. Imagine a solution with two projects: A (ASP.NET web application in C#) & B (regular DLL in C#). A references the project B. Therefore, B's DLL gets copied over to A's bin folder when rebuilding the full solution.

• If you build project A, it will also build all of it's dependencies first. Hence, both get rebuilt.
• If you build project B, only B is rebuilt.

We've noticed that when rebuilding B, an updated DLL appears in A's bin folder. This surprises me, as I thought A would not be touched. This seems like an awesome feature, as it reduces recompilation time when A isn't changed. Is this a new feature? If so, where can we read up on this and other compilation-time magic that was included?