From Matt Cutts:
Google’s Webspam team is working with our Sitemaps team to alert some (but not all) site owners of penalties for their site. In my world (webmasters), this is both a Big Deal and a Good Thing, even though it’s still an experiment. Sign up for Sitemaps to try it out. Oh, and the Sitemaps team introduced a bunch more new and helpful features too. Check out the
Sitemaps Blog for more info.