This is an interesting article discussing how an Agile process can be applied to UI design. One thing to note about what is described is that the whole Agile process, 3 week iterations, daily meetings, etc., is done for the interaction design phase prior to hading something over to development. This is an approach that I have found works well. The thing everyone must remember is that, like in development, you must be willing to "course-correct" as necessary as requirements become more clear or outright change.
This is posted on Alan Cooper's firm's site. No surprise that they know what they are doing. For anyone interested in Alan Cooper's approach, I highly recommend reading About Face 3 - The Essentials of Interaction Design.