I've been following the TDD stuff as an outsider, and hands down the article that makes me consider it with the least skepticism is this one: http://blog.objectmentor.com/articles/2009/10/07/tdd-derange...
I'm sure there are issues w/ the various studies it cites, but it is the only one to actually address TDD from a productivity standpoint with some research data that I've seen. (More / better links welcome too!)