That's why there's a lot of "Lite" versions of games and apps in the Store. It'd be great if Apple added a shareware option in the store - e.g. let the developer set a price and the number of days before the user has to pay to continue using it. Shouldn't even be very hard to implement, since they've got pretty much total control of what can run on the devices already (if we ignore the fairly small percentage of jailbreaked devices, but they can run pirated apps anyway).