Where is the difference when creating the same app for iPhone and then iPhone4?

Is anybody able to explain how do you create the same app for the two versions of the iPhone? I have been readin开发者_StackOverflowg the Developer Guide and a couple of books but I can't figure where should I start at. The only difference i found is in the screenshots' sizes but really, what should I do different when buidling my app: - is there a different process or feature in the SDK or Interface Builder? should i start building it for iPhone3, or iPhone4? Is it the same thing? How would I be able to make it available for both versions at once? Any feedback would be great. Thanks!

Although you can optimize graphics for an iPhone 4, you really don't have to do anything (except maybe provide 2 or 3 icons). An app for an iPhone 3 will run on an iPhone 4 with no updates or changes or new default images. I have tons of old apps on my new iPhone.