A few days ago, Joi Ito blogged about agile design and development:
The problem is, in the real world, things change and by the time you’re done, you’re often pretty far off the mark and usually the first version isn’t right anyway – so you end up making something 2 years late and a hundred features off target. With agile development, you test, evolve and stay in tune with your users and let them guide you. You can also test and refactor more easily because each "story" or feature is smaller, tested and easy to isolate and remove/change. (Or should be.)