Automated acceptance tests are specifications in an agile method. Check out Dave Churchville's blog entry on this subject. Quote:
... Now, for the next iteration, Bill gets a little preoccupied with some nuisance lawsuit, and isn't able to spend as much time with the developers. Since they can't really talk to him, they make some educated guesses about the features, and try to keep it simple.
So Bill has asked for some online help in the next iteration. Our cheerful developers build a maniacal animated paperclip, which certainly fulfills the high level story. Millions of innocent people are made to suffer needlessly.
What if Bill had just written some acceptance tests to clarify things?...