Possible, I believe I've found the offending issue with my sitemap.
The category timestamp is different than the others
<sitemap>
<loc>http://www.coralseait.com/attachment-sitemap.xml</loc>
<lastmod>2013-06-22T00:58:11+00:00</lastmod>
</sitemap>
<sitemap>
<loc>http://www.coralseait.com/category-sitemap.xml</loc>
<lastmod>2013-06-23 13:04:02</lastmod>
</sitemap>
<sitemap>
Notice the different format, and this is indeed the line google complains about. I tried updating a category to force a new timestamp which did happen, but still in the wrong format.