How can I turn off the 3G connection? I'd like to be able to turn the 3G connection on and off without navigating the "Settings -> Wireless and networks -> Mobile Networks" menus each time.

Is there a widget (or some other way) of turning 3G on and off from the home screen or the "Dashboard"?
I use the Power Control Widget. Allows you to toggle Data, wifi and various other settings from your home screen.