These are pretty much the only options available to devs:
1.) Convert existing iPhone app into universal app, makes iPad version free to existing iPhone customers because when they install it, the iPad version will launch by default
2.) Make a new app, they have to buy it like any other app, no way to tie it to another app they have purchased. If this weren't the case, then devs wouldn't have to make a new app when they want people to upgrade their existing app. See MLB at Bat. They can add new features, that are already in the software and unlocked via in-app purchase. See Navigon My Regions.
What I think would be neat is if someone installs an iPhone app on their iPad, when they launch it it pops up with a message for in-app purchase to unlock iPad features for a fee. That would be ideal, and a way for universal apps to take hold, as they would get money from it.
I've also read about some devs who are making universal apps at a loss simply because they think their iPhone app looks horrid as a pixel-doubled iPad version and want the best possible experience for their customers. Fine by me!
