Which comes first, the product or the marketing?
Well, if you define marketing as advertising, then it's clear you need the product first (Captain Crunch being the only exception I can think of… they made the ads first.) This great clip from Mad Men brings the point home. If the Kodak guys hadn't invented the Carousel slide projector, Don Draper could never have pitched this ad.
Marketing is not the same as advertising. Advertising is a tiny slice of what marketing is today, and in fact, it's pretty clear that the marketing has to come before the product, not after. As Jon points out, the Prius was developed after the marketing thinking was done. Jones Soda, too. In fact, just about every successful product or service is the result of smart marketing thinking first, followed by a great product that makes the marketing story come true.
If someone comes to you with a 'great' product that just needs some marketing, the game is probably already over.