Ovid on TDD

Hacker News just gave me a pointer to a nice post of Ovid's on test-driven development. "Many people realize that exploratory programming and TDD don't always play well together." That's a lot of my issue with it. More often than not, when I program I'm looking for the right approach to interfacing with someone else's code or file format. It's very hard to create meaningful unit tests ahead of time in those circumstances, because you don't know what the right answer is ahead of time.