After successfully connecting to a wireless network, the network and network settings are saved on your iPad. If wi-fi access is on, when returning within range of a wireless network you have successfully connected your iPad with, your iPad will automatically connect to the wireless network, or it should. If 3G access is on, when leaving the range of an available wi-fi network you have access to, it will switch to AT&T's cellular network automatically. If both are on, your iPad should switch between the two automatically and seamlessly. Your iPad will always connect to the fastest wireless connection available at the time as long both options are enabled. Between an available wi-fi network that you have access to and are in range of and AT&T's cellular network, it will or should be the wi-fi network.
Page 10 of the iPad user guide regarding status bar icons and what they indicate.
http://manuals.info.apple.com/enUS/iPad_UserGuide.pdf
