One of the features of the new iPhone 3 OS that I discovered quite by accident is the ability to “undo” typing.  Just give the phone a quick shake when you’ve typed something you don’t want.  The “Undo Typing” dialog will slide onto the screen.

If you happen to shake the phone when there is nothing to undo then the “Nothing to Undo” dialog will slide onto the screen.

This handy feature isn’t available everywhere in the phone.  For example, the keypad used for actually dialing telephone numbers does not support undo.