I don’t know many details right now, but ThoughtWorks just released something called Project White as a new toolkit for test automation of WinForms and WPF user interfaces. It might be worth checking out. It’s released as OSS under the Apache 2 license.
I’m happily doing automated testing of my new WinForms application, but I’ve purposely built that automated testing support in from the get go (I build a Window Driver into every screen and use some IoC magic). I’ll write up my solution as part of Build your own CAB soon, but I think most people will prefer something like Project White instead.