Q: Will iPod and iPhone apps that have been updated with hi-res graphics in support of the new Retina display of the iPhone 4 automatically run fullscreen on the iPad?
A: The short answer is no. Firstly, apps that have been updated for the iPhone 4 must still retain backward compatibility with older iPhone devices and their 480×320 screens, and the iPad will still render these in their lower-resolution format by default. Thus far the only apps that have appeared on the App Store specifically designed to run only on the iPhone 4 do not run on the iPad at all.
The other thing to keep in mind is that with the exception of graphical apps such as games, many apps are simply providing higher-resolution versions of specific artwork and icons within the app, relying on the iPhone 4 to scale other user interface elements to the higher resolution.
That said, however, although iPhone apps will still run in a 480×320 window by default, those apps that have higher-resolution graphics for the iPhone 4 should look better when scaled up to the “2X” size on the iPad, although they will be of a noticeably lower resolution than a native iPad app.