A couple of the new software features from the upcoming iPhone 3.0 firmware will only work on the new 3G S model, and those are specifically, support for voice control, new camera controls, video capture, electronic compass, hardware encryption of all data and Nike + iPod support.
The voice control allows for, as expected, voice dialing. The new camera features include "rapid shutter, low light, macro and autofocus, touch controls for automatic refocus, exposure and white balance settings." The camera will also be 3MP.
Video capture will allow for trim editing and easy uploads to MobileMe and YouTube. The hardware encryption will encrypt all internal data, making remote wipes instant. The compass, well, shows what direction you are facing on a map display.
The 3G S will hit stores on the 19th.