Having done what you're thinking about doing, I could probably write pages on the pros and cons, but I'll boil it down to probably the most important reason:
It's hard to go at it alone.
I say that if you don't have a solid cofounder who has your back, an idea and implementation that you're willing to bet your career on, or an extraordinary opportunity that won't be there when you graduate, you should get your degree first and hack in your spare time until then. If you don't have connections, a job with your name on it, significant coding experience, a compelling product, or funding, you're going to have a hard time finding work or even in convincing people to work with you.
That said, programming jobs and startups are much more forgiving in the education category than just about any other job. If you've got the skills and have something to back it up, then you've definitely got a shot.
It's not impossible to succeed, but if you don't have a way to support yourself, it's that much harder. Just know what you're getting yourself into. While you will learn a lot if you drop out and do a start up, know the opportunity costs of what you are going to do.