[gnicholas]

You could incorporate, both contribute all of your valuable IP and whatnot, and receive shares on a vesting schedule. As part of incorporation, you establish ground rules for number of hours spent on the project or milestones to hit, such that either of you can be let go (and halt vesting) if the targets are not hit.

This will not be inexpensive to set up (a couple thousand in legal fees), but if you're ready to launch and want to clean up your legal situation, it could be worthwhile.

Right now you would likely be considered to have a partnership, and you each have rights under that (which vary from state to state). If you go off and build this on your own without a document that your friend has signed, you'll be on shaky ground if he/she comes back after you've become successful. For what it's worth, I am a former lawyer (but never specialized in this particular area of law).