In order to conform to the standard date for RSS, it's necessary the date format of:
D, d M Y H:i:s T
When Drupal is not configured for English, the above date format produces a non-conforming date for RSS feeds as "Mon" for Monday in English then becomes "Lun" in Italian for example. Ideally, the Date module would provide a number formatted for DATE_RSS, but as there are 500+ issues in the queue, I don't think it's going to be done anytime soon.
Can we look at a way of implementing this feature into Views RSS, allowing for international sites to also use this excellent module?
Here is a quick hack I implimented recently: (the format for the date I am using does not have the time, just the date so I filled in the time with 17:00:00
// Expects a date begining with the formatting: YYYY-MM-DD (2010-10-28)
$timestamp = mktime(17, 0, 0, substr($item['pubDate'], 5, 2), substr($item['pubDate'], 8, 2), substr($item['pubDate'], 0, 4));
$item['pubDate'] = date(DATE_RSS, $timestamp);