Both the iPad WiFi and iPad 3G were revealed in the FCC system today; timed perfectly for the start of device pre-orders.
Unfortunately the photographs (both external and internal) and the user manual are all still under confidentiality.
Both units were tested for WiFi 802.11a/b/g/n and Bluetooth and the iPad 3G model number A1337 adds in GSM 850/1900 and UMTS 850/1900.
iLounge notes that the Wi-Fi + 3G model is 0.3mm taller and 0.1mm wider than the Wi-Fi only model most likely due to the 3G models top antenna panel. There is also a 0.1 pound difference in weight between the models.