It looks legitimate. Apple has reported it lost and disabled the phone shortly after it was lost. The level of integration makes it highly unlikely anyone could have faked this. Apple has never intentionally leaked a product that I know of. In fact, they go to extreme lengths to keep all their new products a secret.
• Front-facing video chat camera
• Improved regular back-camera (the lens is quite noticeably larger than the iPhone 3GS)
• Camera flash
• Micro-SIM instead of standard SIM (like the iPad)
• Improved display. It’s unclear if it’s the 960×640 display thrown around before—it certainly looks like it, with the “Connect to iTunes” screen displaying much higher resolution than on a 3GS.
• What looks to be a secondary mic for noise cancellation, at the top, next to the headphone jack
• Split buttons for volume
• Power, mute, and volume buttons are all metallic
• The back is entirely flat, made of either glass (more likely) or ceramic or shiny plastic in order for the cell signal to poke through. Tapping on the back makes a more hollow and higher pitched sound compared to tapping on the glass on the front/screen, but that could just be the orientation of components inside making for a different sound
• An aluminum border going completely around the outside
• Slightly smaller screen than the 3GS (but seemingly higher resolution)
• Everything is more squared off
• 3 grams heavier
• 16% Larger battery
• Internals components are shrunken, miniaturized and reduced to make room for the larger battery