I love this plugin, it's great and I haven't any problem / bugs so far.
But recently I installed W3TC plugin for caching and CDN. I noticed that W3TC gives me additional HTTP header like these:
curl -I http://www.domain.tld/post-429/ HTTP/1.1 200 OK Server: nginx Date: Thu, 23 Aug 2012 23:46:05 GMT Content-Type: text/html; charset=UTF-8 Connection: keep-alive Vary: Accept-Encoding X-Pingback: http://www.domain.tld/xmlrpc.php Link: <http://www.domain.tld/?p=429>; rel=shortlink Last-Modified: Thu, 23 Aug 2012 23:46:05 GMT Vary: Cookie Expires: Fri, 24 Aug 2012 00:46:05 GMT Pragma: public Cache-Control: public, must-revalidate, proxy-revalidate
You can see that there is "Last-Modified" tag, which is almost always new even if the post was written in 2006. I believe it's something good, Google bot will crawl that page or site more often.
But you also know that Google XML Sitemap gives the truth about when the post is updated (in "LastChange" column) which is 2006.
What do you think about this? Having "LastChange" in sitemap and "Last-Modified" in HTTP header together, but the date is not same / in sync.