How do I get a diagram of the dependencies between my C# projects

You could have a look at NDepend : http://www.ndepend.com/

edit to add:

Patrick Smacchia, lead developer of NDepend, blogs here: http://codebetter.com/blogs/patricksmacchia/default.aspx and has written much on the subject of componentisation, which you may find useful.

If you have access to Visual Studio 2010 Ultimate, you can select Generate Dependency Graph from the Architecture menu.

There is a walkthrough on MSDN: How to: Generate Dependency Graphs for .NET Code