Ok; that'll solve the chicken problem -- do you need people for the egg side of the equation? I, for one, would be interested in providing free feed back on iPhone apps. (150 - 200 words per app.)
Currently you don't have to provide feedback in order to get apps for free on the service but I am currently exploring a service that provides usability/feedback for a fee from app developers. We will see how it works out and if there is a market for this soon.