I think there is a bug on this control.
I see no possibility to handle if no button is selected.
After pressing the selected button, the selectedIndex should be set to -1. (toggleOnClick="true")
if you add the following line on onClickHandler event:
it does not display "-1" if case the button selected was pressed. (and no other button is selected)
Am I doing something wrong?
This was a recognized bug that has been fixed in the newer releases of the Flex 3 SDK. You can go here to get a newer version of the Flex SDK: http://www.adobe.com/products/flex/flexdownloads/