I love this post from Guy Kawasaki: Is a Business Plan Necessary?
For all but the most incremental of innovation efforts, a comprehensive business plan is shot in the dark. You’re guaranteed to be 100% wrong. So why try to be 100% right and successful in planning a business venture, when what the humans who will make or break you really only care about something which is 70% "good" execution? Don’t get me wrong — a business plan is really valuable as an exercise in logical thinking. But to mistake it for an exercise in producing a tangible reality is to build castles in the air.
Don’t waste your time. Build to think. Just do it.