According to Jakob Nielsen (and probably popular wisdom too) iPhone apps are downloaded a lot more than they are used. However, the charge for a non-free app is incurred when it is downloaded. Taking this to its logical conclusion, app writers are more likely to design to entice you to download rather than to get you to keep using their app. Can this be fixed? Possibly.
Since Apple controls the whole platform, they could gather usage statistics from each user and app, sync them when the iPhone is connected to the computer and gather those statistics in aggregate at Apple HQ. Given this world-spanning database of app usage, app writers could be compensated by the amount of continuous use their app gets.
Users wouldn't want to pay for the time spent in an app, of course, so the pricing model would have to change. Instead of charging for downloads, the iPhone plans would include an app service charge that provides perpetual free access to anything in the app store. That service charge is then broken up and distributed to the app writers based on their usage stats. Now app writers have incentive to write for continued use (which should equal good design and desirable functionality) rather than downloads.
Mokalus of Borg
PS - The service charge needs to be negligible or very subconscious.
PPS - And it would be hard to switch to this model after the pay-per-download model.