So as Zarhym says, he'll certainly have a presence both as we level and at the farthest endgame content (his presence will at least be felt everywhere, even if he's not standing right in front of you). Medievaldragon suggests we'll see him as Daval Prestor some more, but Deathwing seems almost completely lost to rage -- my guess is that we'll see more of his servants in the Black Dragonflight poking around in human form rather than the main man himself (plus, by now you'd think anyone with the last name "Prestor" would send red flags flying in the Alliance anyway). At any rate, it's sure to be a much different experience than what we've seen of Arthas so far -- and I can't wait.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgen to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.