Over at Lee Goldberg's A Writer's Life, there is a discussion regarding the pros and cons of writing your novel from an outline. My take is this: Starting your novel with an outline is like starting your trip with a map. While you're on that trip, you have the option to stick to the plan, or to take any interesting detours you like. But without the map, who knows where you'll end up?
(To many an unpublished writer, "who knows" sounds like some magical, wonderful place. But you need to write your fiction within certain parameters to have even a shot of making a sale -- trust me, you want to have some rough idea where you're going. Even if should happen to change your mind along the way.)