[WordPress] Code from our blog post How to get Gallery Images in WordPress 3.5 - http://www.limecanvas.com/how-to-get-gallery-images-in-wordpress-3-5/

code-snippet-1.php

/** Grab IDs from new WP 3.5 gallery **/
function lc_grab_ids_from_gallery() {
global $post;

$attachment_ids = array();
$pattern = get_shortcode_regex();
$ids = array();

if (preg_match_all( '/'. $pattern .'/s', $post->post_content, $matches ) ) { //finds the "gallery" shortcode and puts the image ids in an associative array at $matches[3]
  $count=count($matches[3]); //in case there is more than one gallery in the post.
  for ($i = 0; $i < $count; $i++){
    $atts = shortcode_parse_atts( $matches[3][$i] );
    if ( isset( $atts[ids] ) ){
      $attachment_ids = explode( ',', $atts[ids] );
      $ids = array_merge($ids, $attachment_ids);
    }
  }
}
return $ids;

}
add_action( 'wp', 'lc_grab_ids_from_gallery' );