After 10 years of building web businesses, I've learned one thing... Business Plans are ALWAYS wrong! I defy anyone who says they have executed their business plan perfectly from the start. The truth is that there as so many factors you cannot predict when building a business, so why try? Is it an excercise of the mind? A personal challenge? Why do so many entrepreneurs spend so much time writing business plans when they're always wrong?

Obviously the core of what you're planning on doing should be outlined, but a business plan is only needed if you're going to go off and raise capital or get a loan. Typically they only want to see you've thought your business through and it makes sense to them. They'll also be judging you based on your experience as to whether you can actually execute on your business plan as well. 

Here's a quick video of Marc Andreessen giving his viewpoint of what a business plan means to a Venture Capitalist.

 
So in other words: Your business plan does nothing other than force you to think and prove that you've thought about the full details of your business.
 
It's definitely not a bad thing to think about your business in depth. Just don't expect to be right and definitely don't be so rigid that you can't change it at a moments notice.

This is a serious subject that I see many entrepreneurs spending a lot of time on. It's up to the entrepreneur to decide how far they want to take their business plan.