After some testing I noticed that if I selected a value, disabled the radio, then submitted the page item's value would be null (empty string).
You can view an example of this here: http://apex.oracle.com/pls/otn/f?p=20195:2200
They're several ways to fix this problem. The easiest would be to "undisable" (i.e. re-enable) the item before the page is submitted. I know "undisable" is not a word but to re-enable a page item you need to call the disable function: $x_disableItem('PX',false);