There isn't a whole lot to say about the following video. It pretty much speaks for itself. However, we could lay down some speculation.
The first thing I thought of was the possibility that, in order to summon Arthas to kill him, a raid must return the Frostmourne to him. There's a bunch of lore stuff involved there though and I'm not going to get into it.
The second thing I thought of was that maybe this video is showing someone becoming the Death Knight class. I could see it. Roll a character, watch a small gameplay cinematic, and then appear at the Frozen Throne ready to touch the Frostmourne and become a Death Knight.
The third thing I thought was that this could just be a bunch of bullcrap that was filmed on a private server.
Whatever the video was about, it was weird. Very, very cool, but weird. The title of this post included a question mark because I can't confirm as to whether this is an actual leak or not. I honestly don't know what it is... >.>
I also wanted to post the following video. I don't think that it's necessarily new, but it shows some cool WotLK content. The video is of a rogue running through Utgarde, the first 5-man dungeon in WotLK. It was taken at GenCon 2007.