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Yeah Matt is right they have been doing this for a while, they also change the description too sometimes.

As Mihai noted, the Just Discovered Links is a beta feature that gets it's tweet data from Topsy. So if your tweeted link appears in our Topsy feed, you'll see it here. If you're a PRO member, you can also track recent links and metions through Fresh Web Explorer, which uses feeds instead of tweets for discovery. http://freshwebexplorer.moz.com/

Sadly, we can't make Google display rich snippets. All you can do is follow the best practices of implementing rich snippets, and leave it to Google to algorithmically determine they should appear. For the search phrase "phraseexpander", download.cnet.com is already displaying rich snippets. Therefore, the presence of snippets in the SERPS already may impact whether or not they are shown for your domain too.

I'm currently using XML Sitemap for Videos that generates the video sitemap. Is that enough? I also saw this plug in http://yoast.com/wordpress/video-seo/ and it looks like you can display video snippets even if the video are hosted on youtube. What's your experience? Thanks. Andrea

I personally have not ever felt it would be necessary to do something like that. The only thing that matters is if the search engines see your proper setup and accept it.  So monitor Google Webmaster Tools and how they treat your sitemap files - you say they've already been submitted so GWT should see them - that system will let you know if there's a problem from their view. Whether other sites that are not the big three search engines have a problem is not important to your having them the way you do - they are only files that the search engines need.