Most of you probably follow InfoQ and have seen the recent article referring to the study showing that TDD improves quality. The data seems to show that it is 15-30% slower to develop using TDD, but the defects seem to decrease on the magnitude of 40-90%. Most managers would agree that the defect savings would more than pay for the increased cost. The study had very comparative control subjects, so the data should be considered valid. A peer of mine mentioned that this type of TDD was not as mature as could be, and therefore would imply the gap might be even bigger with TDD mature teams.
Also, Mike Bria made an attempt at creating an elevator pitch to pronounce the values of TDD. His pitch is a little longer than my typical elevator ride, but it is a good explanation for those techies looking for a better understanding.