Cameron Skinner has recently made two excellent blog posts that give a great explanation of how to use Layer Diagrams for architecture and design validation in your code. He shows you the basics of creating a layer diagram and binding it to your solution elements. He then shows you how to incorporate that validation right into your build!
Excellent stuff you should certainly go read.
(For all this stuff, you will need to be running the VSTS 2010 CTP.)