a remark on a code comment

Great plugin!

I have just a minor remark about this code comment:

function apt_check_required_transition($new_status='', $old_status='', $post='') {
    global $post_ID; // Using the post id from global reference since it is not available in $post object. Strange!

I think the reason for this is that you are using

add_action('transition_post_status', 'apt_check_required_transition');

instead of

add_action('transition_post_status', 'apt_check_required_transition', 10, 3);

to activate it, so the input parameters $old_status and $post are empty in your case.

cheers

http://wordpress.org/extend/plugins/auto-post-thumbnail/