I have been asked many times before on projects of varying size. My rule is this: never do it for free even with a percentage on the back side. If my new "partner" doesn't have skin in the game, it's not worth the effort on my part, because he could just take it and go back to playing with his Wii and I'm the only person out anything.

I never do one of these for less than half my going rate (because that's my breakeven point anyway). After that, I get a good contract in place, possibly one that gives me stock in the company as a way to guarantee that whatever happens I still own a piece of it (because the project you create becomes an asset of the company that you would then own stock in, so if the company liquidated, you'd at least get a piece of it).

Hope this helps. Good luck with it.