• 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 960x640 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