Thanks (as always) to Boubouille from MMO-Champion for this news, and to the player who sent it there. You can read the details here, but the basis is: the second wing of Icecrown Citadel with Rotface, Festergut, and Professor Putricide will open the 5th January. So we have 4 weeks (probably a bit less spent raiding with Christmas and New Year during that period) before the next wing is opened and more bosses are given to us.
Boubouille speculated that the next gating will probably be shorter, or we would not have Arthas before April (and hard mode can't be unlocked before you beat Arthas on normal). He's probably right, 2 to 3 weeks is probably the time we'll have to wait during the next gating periods.
Until the 5th, we can have fun with Lord Marrowgar, Lady Deathwhisper, Gunship Battle, Deathbringer Saurfang in the entrance of the Citadel raid. But don't forget the new instances, I think the bosses there will be a fun challenge even for moderately geared players.
More news about the patch when I'll have been able to play it, the servers just came up again in Europe and I won't play before tonight :)
If you're experiencing problems, look at the list of known patch issues also present here. And don't forget to update your add-ons beforehand (like when you're patching ;p).