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.