A tantalizing new trailer made a brief appearance in Games Workshop's YouTube channel last night. It was only available very briefly before it was locked down, but it's contents were seen by several people. What it contained was approximately 38 seconds of Dark Angels artwork, followed by the date you see above. And while the video is set to private now, some screen caps were still taken in the brief time it was active.
These images, along with the one above are courtesy of Mariog on WarSeer.
So, what conclusions can we draw from these? Well, while it might seem like an indicator of the impending Dark Angels codex, it's also important to remember that Dark Angels are the mascot of 6th edition. In the first picture, we can see the mark of Chaos Undivided in the background, and what could be the hand of the new Hell Brute model reaching for the Librarian. This leads me to believe that what we're really seeing is a teaser for the new starter box! :D