9to5Mac has a lengthy article with information from unnamed sources about the new look for iOS 7. They say the design will be "black, white, and flat all over." In other words, Apple will be dropping the shadows and highlights that currently dominate the OS, as well as getting rid of skeuomorphism (designing interfaces so that they look like real-world objects, like pads of paper or printed calendars).
Apple's Jony Ive is heading up the redesign, and his design philosophy is quite different from that of Steve Jobs and former iOS chief Scott Forstall. Ive says that texture-heavy skeuomorphic designs don't stand the test of time and that Apple's current apps all look so different it could be confusing for users.
The new design is expected to be officially revealed at Apple's Worldwide Developer Conference in June.