Plan for yourself, not the VC
There have been a few VCs writing about whether or not you need a business plan to make a venture capital pitch in response to an artilce in the New York Times. As Angelo points out, most VCs focus on the team and the market, not on the detailed plan. I wouldn't advise an entrepreneur to develop a full business plan in order to raise money. Normally, a good PowerPoint presentation with some projected finances is enough formal material. Also, be prepared with a good market and competitive analysis. The VC will learn alot about you from how well you prepare this information.
An early-stage investor knows that the market will change a lot before a new company's product is available. It's impossible to anticipate what will happen. So, most investors focus on how you approach things. How do you react to the suggestion of competitive threats? Are you overly optimistic? How customer focused are you? Do you realistically assess the risks in the plan?
You'll have to do a lot of planning to produce the slides and financials. You will need to understand the market well, even if you don't do formal market research. You can learn from customers and other market participants. You'll have to have a development plan, a sales and marketing plan, and an operational plan. You don't have to pull them all together into a glossy document with lots of prose, but you better be able to talk about it as if you've written one.
And, once you have funding, you'll need an updated operating plan. This will keep your team coordinated, make priorities clear, and make sure you have enough money to get it all done! Don't wait for your investors to ask for a plan. Produce a plan for yourself, and keep it current as the world around your company changes.