I wonder if this is related to the problem I'm seeing at the moment...
Google Webmaster tools reports a 404 error on my sitemap and when I go to the WordPress SEO "XML Sitemaps" settings and click the view XML sitemaps button, I also see a 404 error.
Logging in via FTP confirms that all my sitemap files are gone... and I can't figure out how to regenerate them with the WP SEO plugin.