Q: If I own the iPhone app of a game such as NOVA will I have to pay again if I want the iPad version?
A: You will be able to continue using your existing iPhone and iPod touch applications on the iPad without needing to get an iPad-specific version, as the iPad will be compatible with the majority of existing applications available on the App Store. Obviously, however, applications designed to run on a 320×480 iPhone screen may not look as great when run on the larger 1024×768 iPad screen, nor will they be able to take advantage of any other iPad hardware advantages.
As a result many developers will likely be releasing new versions of their applications and games that are designed specifically for the iPad. Developers can choose to either produce a separate version of an application that run only on the iPad, or produce a “Universal” version that supports all of Apple’s current touchscreen devices, taking maximum advantage of whatever hardware is available.
Whether or not you will need to pay for an iPad-optimized version of an existing app is going to depend entirely upon how the developer chooses to release their iPad version. If a developer chooses to release an iPad-only or Universal version as a completely new, separate application it will be a separate purchase as it would be with any new application. However, it is also possible that a developer could choose to release a Universal version as a version update to the existing app. This would be treated as any other application update, and under the current App Store policy would be a free update.