I have to say that I enjoy reading Seth Godin's books. He centers upon a core idea and explores the concept from various angles.
The Dip is about the setbacks we face that we can overcome if we keep pushing. This is opposed to the Cul-de-Sac, which, no matter how hard we try, we cannot overcome. Seth says that the ability to know when to quit Cul-de-Sacs and when to stick with Dips is what sets the stars apart from the average.
It's certainly an interesting idea and one well worth exploring further.