If you ask Bill Gates, Michael Dell, or Larry Ellison, you don't even need an undergrad college degree to start a company...
Put it this way. To start a business, you need equal doses of:
(1) The brains to analyze situations, plan well and make good decisions (Knowledge),
(2) The heart to get through many tough times because you love what you do (Passion)
(3) The guts to simply do it when others are simply "talking" about doing it (Chutzpah).
The MBA can give you knowledge, but it can't give you passion or chutzpah. And what prevents most people from starting their own business isn't lack of knowledge -- it's lack of heart and guts. I'm sure you've met people in your office (every office has one) of the brilliant but chronic complainer who always talks and talks about leaving and hanging up his/her own shingle... but five years later is still talking about it... and then years later still talking about it.... and in many ways it's hard to blame them - starting a business is a romantic notion until you have to do it -- and saying that it's tough is an understatement -- your wallet is always empty, your family is on edge, your psyche is all twisted up, your ego takes a beating, and you can be an emotional wreck since you have so much of your own identity tied up in the biz.
What you learn in an MBA program can certainly help, but it's certainly far from being a dealbreaker when starting a biz -- it's a luxury more than a necessity.