This is a screenshot from iPhone software 2.0 Beta 5 that will show up if you are using an iPhone with 3G (obviously not yet released), it will not show up on an EDGE iPhone. The preference is hidden inside of /Applications/Preferences.app/Network.plist.
The image above seems to reveal that the 3G chipset still may not give the battery life that Apple would like so they are giving 3G iPhone owners (when it is released) the option to fall back to EDGE speeds for increased battery life.
Currently I am expecting an iPhone with 3G to be released sometime between June 15 and July 15.
This information has been confirmed by MacRumors.