From Brett Bim:
Microsoft Research has put out a paper with data that shows implementing TDD ... decreases your pre-release defects by 40-90%.
Paper is here. InfoQ's take:
The pre-release defect density of the four products, measured as defects per thousand lines of code, decreased between 40% and 90% relative to the projects that did not use TDD. The teams’ management reported subjectively a 15–35% increase in initial development time for the teams using TDD, though the teams agreed that this was offset by reduced maintenance costs.