$product_ids = array();
if (($order_id = commerce_cart_order_id($GLOBALS['user']->uid)) != FALSE) {
  $order = entity_metadata_wrapper('commerce_order', $order_id);
  foreach ($order->commerce_line_items as $line_item) {
    $product_ids[] = $line_item->commerce_product->product_id->value();
  }
}

Drupal Commerce 1