In case you haven't heard the background of Twitter, and how Evan Williams and Biz Stone developed the company as a side project, Michael Malone's article, "The Twitter Revolution: The brains behind the Web's hottest networking tool" in the Wall St. Journal is worth reading.
There's a lot of emphasis in the small business publications like Inc., Entrepreneur, FSB, and BusinessWeek SmallBiz, on developing a business plan.
And a lot of emphasis on tons and tons of hard work to execute against that business plan.
Williams, however, started Blogger.com as a side project while at Pyra Labs, as a way to make notes as a kind of project management tool. Blogger.com went on to be sold to Google for millions.
Then they started a new company, Odeo, developed a new business model as a podcasting company. And, once again, developed Twitter as a sideline. Which took off.
I guess the point here is that business plans are important but you have to be able to adjust.