alt Hacker NewsSDD is about flowing the design choices from the spec into the rest of the system. TDD was for making sure that the inevitable changes you make to the system later don't break your earlier assumptions - or at least warn that you need to change them. Personally I don't buy TDD - it might be useful sometimes - but it is kind of extreme - but in general agile methodologies were a reaction to the waterfall model of system development.
This is just one way to use TDD. I personally get the most value from TDD as a design approach. I iteratively decompose the project into stubbed, testable components as I start the project, and implement when I have to to get my tests to pass. At each stage I'm asking myself questions like "who needs to call who? with what data? What does it expect back as a return value?" etc.