[jacalata]

If you planned ahead, you could keep a version log somewhere inside the software showing which versions they'd had installed. Or, you could have each new version installed 'phone home', maybe ask users to 'register now for free/cheap upgrades later!'. All of them have drawbacks, but would get a significant chunk of the desired experience.

[timeuser]

The drawbacks are pretty significant. A lot of users really aren't that savvy or motivated to register ahead or deal with some complicated upgrade process. If you have an app that's been in the store for a while it's even more problematic since more users have old app versions. Anyone that gets a new device and wants to install the upgraded app would have to install the old app version or go through some other odd process if they want to activate the upgrade. Also it seems likely Apple won't allow these upgrade schemes any more than they are allowing Omni to do upgrades around the App Store.