Does the iPad support EDGE when 3G is unavailable?

Yes, the iPad Wi-Fi + 3G will switch seamlessly between available data networks including EDGE and Wi-Fi. The receivers inside the iPad support the following frequencies:

UMTS/HSDPA (850, 1900, 2100 MHz)
GSM/EDGE (850, 900, 1800, 1900 MHz)
Wi-Fi (802.11a/b/g/n)
Bluetooth 2.1 + EDR technology

The iPad supports 3G speeds up to 7.2 Mbps (HSDPA 7.2) on cellular networks and will automatically switch to Wi-Fi if it becomes available.