While we haven't been able to independently verify this ourselves, one blog post claims the latest Beta 5 iPhone Network.plist file contains 3G preferences, and that somehow can be activated. The claim reads:

Note that this is not fake, it is in the beta 5 .ipsw, but will normally not show up on an EDGE iPhone. Turning 3G on will just crash the phone if you use an EDGE phone. If you do not believe me, then decrypt the beta5 rootfs dmg, and see for yourself in /Applications/Preferences.app/Network.plist