I am familiar with nunit for unit testing of the business layer however I am looking now to automate the test of the win forms gui layer.
I have seen watin and the
Check out http://www.codeplex.com/white and http://nunitforms.sourceforge.net/. We've used the White project with success.
Same Answer to a previous question
Edit
The White project has moved, and is now located on GitHub as part of TestStack.