Ron Jefferies, one of the inventors of Extreme Programming, and a man I admire very much, is working on an online book, Adventures in C#, of which the latest installment is Adventures in C#: Using NUnit, Be sure to check this out.
I’m reading the tutorial while at work, it looks like an excellent resource. The tutorial made sense to me, having some experience with JUnit on the Java side of things.
Adventures in C# also has some great info for applying eXtreme Programming to .NET.