The plugin is creating the sitemaps fine and Google is parsing them correctly; however I get a /sitemap-pt-post-1999-11.xml file, yet there are no posts with this date in my database. I checked the posts list, I also did a manaual search by doing a select straight from the posts table.
Google Webmaster tools shows
/sitemap-pt-post-1999-11.xml
Errors Sitemap Jan 23, 2012 -- --
Sitemap errors and warnings
Line Status Details
Errors 4
Missing XML tag
This required tag is missing. Please add it and resubmit.
Parent tag: urlset
Tag: url
Problem detected on: Jan 23, 2012
http://wordpress.org/extend/plugins/google-sitemap-generator/