Just in case, like me, you were wondering just what happened in the cinematic for Wrath of the Lich King yesterday (wait -- Arthas puts Frostmourne in the ground and a dragon comes out?), it looks like we're likely to get some answers. We didn't exactly see any familiar player characters or races in the trailer, but apparently the location of Arthas' little dragon summon can be seen in Northrend, as per the shot above. We still don't know what spell it is that summons a frost wyrm, but you can clearly see the hole in the ground the thing left.
And while most players assumed the wyrm was Sapphiron (the frost wyrm boss of Naxxramas), MMO Champion doesn't think so -- there's a model in the beta called Sindragosa (labeled "The Frost Queen"), and there's a place called "Sindragosa's Fall" -- we're not sure where it is in the game, or if that's it above, as Icecrown Citadel has barely been explored.
So maybe further questing and lore in the expansion will give us a little bit of a clue as to what exactly Arthas was doing in that trailer. Odds are that dragon we saw is ingame somewhere, and we players are probably just the dragonslayers to slay it.
Update: Yep, the area is Sindragosa's Fall, which makes the dragon almost definitely Sindragosa. So it looks like the summoning we saw in the cinematic might be more story significant than we thought.