Transforms Categories Checkboxes Selection into Radio Buttons in Wordpress.

Category to Radio.php

<?php
# paste into functions.php in theme
add_action( 'admin_footer', 'categoryRadioButton' );
function categoryRadioButton(){
	echo '<script type="text/javascript">';
	echo 'jQuery("#categorychecklist input, #categorychecklist-pop input, .cat-checklist input")';
	echo '.each(function(){this.type="radio"});</script>';
}
?>