Expanding the screen without changing the size of the phone was done by shrinking the border around the screen to nothing. The case also will be curved, according to the rumors, unlike the more square construction used in the iPhone 4.
It's also expected that the antenna will be redone--perhaps as part of the logo--after the iPhone 4's side antenna problems. In the iPhone 4, the antenna wouldn't work if the phone was held in one hand so that the hand touched both sides. This led to Apple providing free phone cases to all iPhone 4 owners.
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