Possible Duplicate:
How to accommodate for the different screen resolution of iPhone 4?
What does the new resolution mean for existing applications?
Do the devtools make it easy to support new resolutions without changes, if so how? what happens to apps by those developers who dont do things properly to leverage this?
What are the rules for resolution compliance for app acceptance on the app store?
The new OS should (As Steve Jobs stated during the keynote) detect whether or not the app is built for the new res and scale accordingly. The new resolution is just double the old, so it should be a simple scale-up on the operating system’s end.
All of the current UI elements will support the new resolution once iOS4 is released, so all a developer will have to do is update their custom images.