Octopart, for example, was founded by a bunch of physicists. One is now doing sales. Sales is not the most natural work for a physicist, and maybe one day they'll have official sales guys doing it, but for now he is doing surprisingly well.
Business type qualities can be shared across several non-business types easily. When you see startups without the business type succeeding, its because it is there amongst all of the co-founders, inside of them.
What is necessary is the presence of someone who really understands people. Your product is destined to be used by people; so make sure that someone on your team has an interest in truly understanding your users and the other people that you will have to deal with.
Someone who can read a financial statement or is willing to learn, who spent a lot of time growing up or in school reading about the web business, who has started up a few businesses whatever they do, who is interested in that side of the company: yes (as long as they are technical too).
Someone who is an MBA but has limited real world experience: no.