We plan on releasing the four sections of Icecrown Citadel over time and not all immediately when patch 3.3.0 goes live. At this point I can't give precise dates for these release dates as they are determined by when patch 3.3.0 goes live. Once dates are known with more certainty, I'll update the community so they can plan appropriately.
The first section that opens will include the Lord Marrowgar, Lady Deathwhisper, Icecrown Gunship Battle, and Deathbringer Saurfang encounters. Progress beyond that point will be prevented for several weeks. Then the Plagueworks will open with Rotface, Festergut, and Professor Putricide becoming available. After another period of time, the Crimson Hall will open and you can then fight the Blood Princes and Blood-Queen Lana'thel. The final Frostwing Halls unlock then occurs after that, making Valithria Dreamwalker, Sindragosa, and the Lich King available. We believe a staggered release of the content will allow players to experience Icecrown Citadel at a sustainable, measured, and ultimately more enjoyable pace.
There are other elements that gate access along the way. Players may not attempt any Heroic versions of 10 player encounters until they have defeated the Lich King in a 10 player raid. Similarly, players must defeat the Lich King in a 25 player raid before they can attempt a Heroic 25 player encounter. So players must master every normal difficulty encounter in Icecrown Citadel before attempting Heroic difficulty.
The Lich King may not be attempted until Professor Putricide, Blood-Queen Lana'thel, and Sindragosa are defeated. Furthermore, the Heroic difficulty of The Lich King encounter may not be attempted in any week unless the three aforementioned encounters have been defeated in Heroic difficulty that week.