New to the 4G are a front-facing camera for video conferencing, a better standard camera (including larger lens and flash), a Micro-SIM slot (just like the iPad), 960x640 resolution, a second mic, metallic exterior (not plastic) and split side buttons for volume.
The form factor is very different as well. The back is flat and not rounded, slightly smaller screen, a more "square" design all around with less wasted space, a larger battery, and smaller internal components.
Wal-Mart announced this week that it is slashing the price of the iPhone 3GS in half, with the 16GB 3GS model going on sale for $97 USD with two-year contract. The smartphone currently costs $199 through Apple. The move has led many to believe that Wal-Mart is clearing out remaining stock in anticipation of the new iPhone launch.