I am relatively sure it couldn't be done as a startup. The profit margins just aren't there. My back of the envelope math says that each Work Unit, which, based off my experience takes ~8 hours to finish, would go for ~$0.80 (assuming that EC2 does a comparable speed, which isn't guaranteed). In order to make a decent profit, you'd have to be paying the users <$0.20 per unit, or roughly $0.60 per day, per core. It doesn't seem like a good enough proposition, that will barely cover the increased power consumption.

On the other hand, if an existing company branched out into this, something like Amazon and MTurk, I could see it working. A company which already has a user base, which has software installed on clients could leverage this as a "Oh yeah, here, we think this is cool, try it".