I don't agree with everything in 37Signal's new book Rework, but I agree with what they say about business planning:

Unless you’re a fortune- teller, long- term business planning
is a fantasy. There are just too many factors that are
out of your hands: market conditions, competitors, customers,
the economy, etc. Writing a plan makes you feel
in control of things you can’t actually control.
Why don’t we just call plans what they really are:
guesses. Start referring to your business plans as business
guesses, your financial plans as financial guesses, and
your strategic plans as strategic guesses. Now you can
stop worrying about them as much. They just aren’t
worth the stress.
When you turn guesses into plans, you enter a danger
zone. Plans let the past drive the future. They put
blinders on you. “This is where we’re going because,
well, that’s where we said we were going.” And that’s the
problem: Plans are inconsistent with improvisation.
And you have to be able to improvise. You have to
be able to pick up opportunities that come along. Sometimes
you need to say, “We’re going in a new direction
because that’s what makes sense today.”
The timing of long- range plans is screwed up too.
You have the most information when you’re doing
something, not before you’ve done it. Yet when do you
write a plan? Usually it’s before you’ve even begun.
That’s the worst time to make a big decision.