Interesting question. I have no idea.
I don't know that I would pick a friend, or at least a real close one. Friendship, or rather mere compatability, is not always the best attribute in a business partner, and I've often heard it said that you shouldn't go into business with your friends. I would look more for a colleague/acquaintance whom I have been able to work well with in the past, whose judgment, work ethic, and opinions (although not necessarily identical to my own) I respect, and who, most importantly, I could absolutely TRUST. Tall order, I know, but like with a roomates, friendship can complicate a partnership.
Or maybe all my friends are just rubberheads.