The technique described in Luzian Scherrer's blog post does work in iOS 4.0.
My code to add the button to the keyboard was failing because it was being called from the textFieldDidBeginEditing: delegate method, which (in 4.0) is called before the keyboard window's subviews are created. I fixed the problem by calling my code when UIKeyboardWillShowNotification is observed, which happens late enough.
