What's new
• 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 960x460 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

What's changed
• 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