Thomas Vander Wal has an interesting post about quick and intense usability iterations:
I definitely see the strong advantages of the intense sessions mixed with the usual longer term development. Finally it seems a broad section of the development world is finally learning that the best way to build out stuff is to sit with the people that use it, see their pain and frustration. But, even better is fixing that pain overnight. These intense iterations build positive feedback for the developers and designers on the projects, the business owners seeing quick improvements, and the people who want and need to use the products. The people using the tools will most likely go away and become evangelists for the products as the developers and designers not only listened to their needs, but fixed it so it worked better for them right before their eyes.