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Google announced that if you want to do content discovery right by Google, do not just use an XML sitemap file but also submit an RSS/Atom feeds.
Google's Alkis Evlogimenos wrote, "for optimal crawling, we recommend using both XML sitemaps and RSS/Atom feeds." Why? The XML sitemaps will give Google information about all of the pages on your site while the RSS/Atom feeds will provide all updates on your site, helping Google to keep your content fresher in its index.
Google also recommends you notify Google of new URLs via your files by using both sitemaps pings and PubSubHubbub. Of course sites that are indexed quickly, like news sites, probably do not need to use sitemaps pings and PubSubHubbub but it can't hurt.
For more technical details see the Google blog.
Forum discussion at Google+.
