Reading Scott Hanselman's blog I came across a link to Moq, which is a mock object that uses the new C# language features in order to do it's goodness. It looks like a pretty amazingly easy to use mock framework for your unit tests that allows you to set return values from your mock objects by setting conditions through Linq. For example (shamelessly taken from their examples):
// ShouldExpectCallWithArgument
var mock = new Mock<IFoo>();
mock.Expect(x => x.DoInt(1)).Returns(11);
mock.Expect(x => x.DoInt(2)).Returns(22);
Assert.AreEqual(11, mock.Instance.DoInt(1));
Assert.AreEqual(22, mock.Instance.DoInt(2));
Which is all that is needed to create a function that returns specific values based on the input, and there's a lot more power locked away when you start getting creative with that Linq syntax.
I think try this on a real project, it'll be interesting to see how far it can be pushed before you have to try a more complex solution. I suspect it'll cover most things unless you get too crazy.