The iPad 3G wasn’t pictured at the event but is in Apple’s gallery. The difference is obviously the black patch on the back which also comes over the top of the device until it hits the screen. The plastic allows for better 3G (and GPS?) signals to reach the internal antennas.
It will be interesting to see how good Wifi access is on the regular iPad compared to the one with the plastic backing. Keep in mind the iPod touch has a patch of plastic on its back to help the wifi signal reach the internal antenna.