Players pressed their way into Icecrown Citadel and soonly took down the first bosses in the lower areas. After taking down the Deathbringer, talking to your friendly faction hero yields some information as to when the doors will open. In Trial of the Crusader, one new boss opened every week. In Icecrown Citadel, it seems like they're increasing the time between gates. As reported by Stratfu, the next area will open in 28 days (which is expected to be the Plagueworks containing Festergut, Rotface, and Professor Putricide). January 5th is the target date for the next set of bosses.
At the rate we're going (28 days between gates), we won't see Arthas until April.