I'm assuming we are talking about a blend of full-time and part-time cofounders. Let's say you are the full-time cofounder. Before you take the plunge, you better make sure you find out if your "part-time" cofounders will ever plan on quitting their full-time jobs. If they say no to you, will you be prepared for the day they decide to quit?

They will never be as devoted as you will be. They will never be as committed. Sure they may be talented and will write tons of code for you - but what happens if they leave and you have to figure out how all of their code worked?

I'm sure you've read all the articles written by other entrepreneurs about how much dedication it takes to build a company. There are always exceptions to the rule (del.icio.us), but I'd say most successful startups require your full attention.