Disclaimer for forum participants: This is a comment intended for Help users, NOT a request for help.
Note that it doesn't seem to be possible to go back to a "no selection" state, even if you set selectedData and selection to null. An example of why you might want to do this is a multiple choice question where you reset the data with each question, rather than creating a completely new View instance. To work around this, you can create a hidden radio button with a value of 'null' and set the selectedData to 'null'.
Also, the direct link to the Language reference (since searching for RadioButton will give you mx.controls.RadioButton, even if your filter is set to filter out Flex) is http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/fl/ controls/RadioButtonGroup.html .