At Storytellers Unplugged Justine Musk has an interesting post about outlining.
She sets up an interesting continuum between the Outline and what she calls the Undermind, the instinct a storyteller has that tells him or her where a story is supposed to go. Needless to say, the two don't always agree. Because of that, people disagree about the value of an outline. Justine herself is trying to do a novel "without the net" for the first time.
As you can see from my bio, I'm a math teacher. One thing you are taught early on as a teacher (or else learn the hard way) is that you're asking for disaster going into class without a lesson plan. That's not to say it never works out- occasionally you pull something out of your ass and everything goes great. But in general, you always have a plan.
But expecting the class to go exactly as you planned it can get you in as much trouble as not having a plan. Maybe the students will get what you were trying to teach them right away, and you need to come up with something else. More often, the example that you thought would be completely obvious to them leaves them looking at you like you're speaking Chinese, and you have to come at it a different way.
That is my approach to outlining now. I have an outline when I go in. But I'm not surprised when I have to diverge from it, and come up with something new. Also, the outline just marks "milestones" in the book. I still need the Undermind to tell me how to get from milestone to milestone.