The problem you'll find is that as soon a radio button is clicked its state is changed before you can check it. What I suggest is to add a custom attribute to keep track of each radio's previous state like so:
$(function(){
$('input[name="rad"]').click(function(){
var $radio = $(this);
// if this was previously checked
if ($radio.data('waschecked') == true)
{
$radio.prop('checked', false);
$radio.data('waschecked', false);
}
else
$radio.data('waschecked', true);
// remove was checked from other radios
$radio.siblings('input[name="rad"]').data('waschecked', false);
});
});
You will also need to add this attribute to the initially checked radio markup
See demo here : http://jsfiddle.net/GoranMottram/VGPhD/2/