The current J2SE 5.0 spec states the following for the selection/de-selection in
the button groups:

"Initially, all buttons in the group are unselected. Once any button is selected,
one button is always selected in the group. There is no way to turn a button
programmatically to "off", in order to clear the button group. To give the
appearance of "none selected", add an invisible radio button to the group and
then programmatically select that button to turn off all the displayed radio
buttons. For example, a normal button with the label "none" could be wired to
select the invisible radio button."

However, it is not described what would happen if the selected button is
programmatically removed from the button group. The current J2SE 5.0 RI
behavior does not show that the selection is still forced after a button
removal. The spec needs clarification for this.