I've found the opposite, that TDD helps me most when the spec or my understanding of the spec is fuzzy, by forcing me to make decisions about behaviour up-front and clarify my thinking. Otherwise I can get bogged down trying to implement and specify a feature simultaneously, or spend a lot of time implementing a feature before realising I'm approaching it the wrong way.
Depending on how you work, that can be worthwhile, but I usually find I get more useful information quickly by making the decisions that make implementation easiest, getting something up on the screen that I can react to as a user (or put in front of a real user), and then seeing where that initial proof of concept falls short. In general, I've found that pushing off decisions until I have as much information as I can tends to result in better decisions.
