Saw this post by Craig Andera about Test Driven Development and I have to say I completely agree with him.
I've been doing TDD for several years now and I tend to restrict it to testing business and data access layers. Currently, it's not practical to perform comprehensive TDD for UI layers (though tools like NUnitAsp and NUnitForms help).
Even these tools don't address one of the biggest challenges when it comes to testing a UI layer. The UI layer is the layer most likely to change and change often. After spending a few hours building your tests, some guy in marketing will call you up and say, "can we replace that button with a table of data with clickable rows?" What now?
Unit tests tend to be quite fragile when faced with a changing UI layer. Human testers have no problem dealing with such change, but your unit tests definitely will.
My recommendation for testing the UI layer are combining test scripts (for human testers) along with writing unit tests when a bug is found in the UI layer. At that point the UI should hopefully be frozen enough that writing a unit test that exposes a bug and then fixing the bug will be a worthwhile investment for regression purposes.