Several webmasters are noticing new errors found in their Google Webmaster Tools Sitemaps error diagnostic section. The new error is:
All the URLs in your Sitemap are marked as having dynamic content.
No one knows exactly what makes this error spring up. We do not have a response from a Googler on this error, at least not yet.
Google Groups member, Phil Payne, has a nice theory:
As an aside - Google thought this scheme up and defined 'always' all by its little self. So - assuming they meant to do it all along - the only meaning of "Because dynamic content is difficult for search engines to crawl and index, this may impact your site's performance in search results" that makes sense is the implication that putting incorrect information in a sitemap might result in some sort of penalty.
So it's becoming more important to make sure that a sitemap truly reflects the site - most of the sitemap generators need a fundamental redesign.
I will keep watching and let you know when Google does respond.