Or Pay-per-use? I don't know if Apple would even bother with that, or at least with the smaller value apps. I don't think Apple wants to bother charging your credit card for 25 cent rental.
if that's the case they should just make all of the app's in 2 versions one as a free demo (witch is only some not all apps) and full version paid apps.
if you like the demo then buy the full version that's how i see that rolling more.