The iPhone repair site, iResQ, has leaked photos of, what they claim to be, the faceplate of Apple's upcoming 4th generation iPhone. According to the site, the front panel of the phone is approximately 1/4 of an inch taller than that of the iPhone 3GS. The screen is also longer, but exact measurements are not given. In addition, there seems to be some sort of "reflective, mirror-like surface" that sits right above the speaker. It's unclear what purpose this will serve, but iResQ postulates that it could be for the proximity sensor.
The site also notes that the LCD is "factory glued to the digitizer," similar to the first generation iPhone. This will make screen repairs pricier due to the need to replace two parts instead of one. No other hardware details are available at this time.