Google Sitemaps has been renamed to Webmaster Central. Basically, it will be the place for Webmasters to check the health of their sites in Google. Danny has more details here and so does the Google Blog here. Main difference right now that I see is the way to define your “Preferred domain,” i.e. with or without the www. Other improvements and features there also.
Visit it at http://www.google.com/webmasters/
Forum discussion at Search Engine Watch Forums & WebmasterWorld.