TheTechErra went searching through the code in iTunes 10.5 beta 9, and found an interesting series of strings hinting that Apple is about to allow "rental" of iOS apps. The functionality is referred to as "renting," but it sounds much more like an official implementation of app "trials" that lots of developers have been hoping for. Customers would essentially download these app "rentals" (for free or for a price), and then after a certain period of time, the app rental would either expire, or customers could jump in and buy the full version of the app.
Most developers on the store have worked around functionality like this, either building up two versions of apps (lite and full), or doing things like putting certain content past in-app purchases. But if Apple is finally including this functionality into the official system, then most of those devs will be happy to hear about it tomorrow. We'll have to wait and see.