How to write a business plan for an Internet startup?
pedalpete: I'd start by getting the budget and forecast together, and then write the executive review.
Get some feedback on it, and then pass it to the investors.Go from there. You'll get an idea from the feedback and from what the investors say to know if you should proceed writing the entire plan, maybe they will find some holes in it, etc. etc. 
Then if they still want the full plan in writing, go write it. You'll probably learn a ton just in the process of writing the executive review.