Line 2609 of node.module says

$node->rss_elements = array(
      array('key' => 'pubDate', 'value' => gmdate('r', $node->created)),

If you have a locale other than "en_US" on your server you might get a pubDate that is not a valid RSS PubDate. Your rss.xml is not quite valid according to http://validator.w3.org/feed/ and will not work everywhere.

https://drupal.org/node/812260 offers a couple of solutions (#0 and #4) for the views RSS module. Solution #0 may be the best fit for the node.module?