If you've bought [App X] for the iPhone and the developer makes [App X] for the iPad, should you get the iPad version for free?

We've seen developers like Instapaper's Marco Arment specifically making "universal binaries" so users who bought the $4.99 Pro version can download an iPhone update, install it on their iPad, and get an iPad-specific user interface (UI) as well. (Think of it as two similar apps in one handy download).

We've also seen leaked screenshots have shown "HD" or "for iPad" versions of many popular iPhone apps and games that are being released as separate, all new downloads. In many cases these HD apps offer substantial differences brought about by the bigger screen real-estate and beefier processing power of the iPad and will carry their own price tag and perhaps be more expensive than the iPhone versions due to the greater effort put into them (more pixels, more functionality).

Some users no doubt feel entitled to free iPad versions if they already bought the iPhone version while some developers likewise feel entitled to get paid for the work they put into making different versions for a different device.

Can we find some existing parallels? If you bought a Super Mario game for the Nintendo DS, you don't get Super Mario for the Wii for free. Likewise PSP and PS3 games. Of course, the differences between dedicated mobile gaming devices and their console cousins are far greater than between an iPhone and an iPad.

If you bought a Mac app like iMovie you could install it on both your MacBook laptop and your iMac. Likewise for Windows. Of course, the differences between Mac OS X or Windows running on a laptop or desktop are negligible compared to the iPhone and iPad (there's no separate version of iMovie for MacBook vs. iMac, or Microsoft Word for netbook vs. tower).

Again, the iPad falls in between.

Apple announced at launch that most iPhone apps will "just work" on the iPad, either 1:1 or pixel-doubled to fill the screen. So, developers who want an easy out can just point to that as the "free" iPad version of their existing iPhone apps. We haven't heard great things about pixel-doubling's looks, however, but free is free, right?

What about developers who invest time and money into offering something better? For the most part, however, we agree with TUAW's Erica Sadun who suggested that apps with a similar user experience between the iPhone and the iPad could be good candidates for the universal binary approach (i.e. free but tweaked for the end user). Apps that are significantly re-designed and re-engineered from the iPhone to the iPad, however, could be good candidates for the separate HD app strategy.

As to whether or not developers should offer discounts to existing iPhone users for iPad apps, that one's easy for now -- despite hints and rumors, Apple still has no "upgrade" or "discount" option available to developers. Even if a developer wanted to, they can't offer anything but a single, consistent, full price for everyone. (See the Tweetie 2.0 controversy).

We're one week away from iPad launch day, so things can still change, but until and unless they do, tell you what you think -- should iPad apps be free for existing users? And if people won't pay more for better, what incentive do developers have to make more and better apps?