As of this writing, the feature is only showing up with this particular Editor's Choice listing, and only in the UK. The trailer is also limited to that Editor's Choice page, which is separate from the app's regular page in the App Store. MacStories broke the news.
Clumsy Ninja Editor's Choice Page (video on the right)
It remains to be seen if Apple will bring it to other App Stores and/or make it available for all apps. Likewise, it remains to be seen if the page with the video trailer remains accessible once this app is rotated out of the current Editor's Choice spot. Clicking on past Editor's Choice apps takes you straight to the normal page for that app.
Screenshot from the Trailer
Personally, I hope Apple is in the process of rolling out this feature as a permanent fixture of the App Store. Seeing a video of game play is so much more telling than looking at a static image. In the case of Clumsy Ninja, for instance, the video trailer does a much better job of conveying the quirky nature of the game and character alike.
On the other hand, this will amp up the stakes for developers. If every app could have a trailer, those developers with the resources to make a quality video of their app will be at a distinct advantage to those who don't, skewing the App Store that little bit more towards larger companies.
It will be interesting to see where Apple takes this.