wp_set_post_categories is defected in that if it is passed a an array of categories who's index starts at other than 0, it returns without any action. I noticed this when tracking another issue in Textpattern import.
Example:
$cats = array();
$cats[1] = 10;
wp_set_post_categories($postID, $cats);
This fails because (1==count($cats) && ''==$cats[ 0 ]) == true.
Further, I feel that inconsistent checking behavior needs addressing. Why should this function return true in the single case where it gets an array of values where [0]=='' ?
My suggestion is to use this:
if (!is_array($post_categories) || '' == implode($post_categories))
$post_categories = array(get_option('default_category'));
