I use a 17" MBP as my primary computer, but I see it as more of a luxury than a necessity. I'd say to spend your money on a more modest laptop and save the money for your startup.

On a similar note, I think it's crazy to see all of these startups using Apple Cinema displays. Two examples that come to mind are Color and Milk. Don't get me wrong, they're very nice monitors, but waaay overpriced when compared to monitors with the same specs.