Don't drop out until you have an income from your new business. If you are truly passionate about starting something, and it's obsessing you, you shouldn't need to 'drop out' to get it going.
And even then, when you have INCOME from this endeavor, you'd just be taking a break. It's insurance, really. A degree, however not worth it's weight in paper, will open doors for you.
But so would a hit startup. You could have the next facebook in you. So make it happen, but try, as long as you can, to hold on to the most unique time in your life, the experience of undergrad.
Cherish it and respect it. Leave it when your new venture makes it so clear it's no longer a question you would ask a group.
Leave it when it's obvious.