Allen Morgan has a terrific post examining the question of allocating equity in startups to co-founders. Quote:

If you want to raise money from VC’s, here’s a really tough, really important question you ought to ask yourself very early in the process: “How many co-founders should I have?” Having the wrong “answer” to this question can make your life difficult in some subtle (and odd) ways. Plus, unlike some miscalculations, here the wrong answer hurts only the founders, not the VC or later employees. Here’s how to look for the right answer.