I noticed that you also have an RSS feed which works in the validator.
Have you considered just using that one?
If you are creating the feed yourself, I would recommend using the
native XML sitemap format as described in http://sitemaps.org/protocol.php.
By using the native format you can include all of your URLs and give
additional information to each one (date of last modification, change
frequency and relative priority). The XML sitemap format is accepted
by all search engines which support sitemaps.
When you make changes in feeds that you use as sitemap files, you can
help to make sure that they're processed quicker by resubmitting them
in your webmaster tools account. It may take a bit to get processed,
however, so you should take a look after a few days to check the
status. According to the validator, it looks like it's ok now, great