Blizzard has just released a lengthy statement on the way the new WoW Icecrown raid in patch 3.3 will be progressively accessed. Some of the key points:
Icecrown Citadel is going to be broken up into four distinct sections: The Lower Spire, Plagueworks, Crimson Hall, and Frostwing Halls. We plan on releasing these four sections of Icecrown Citadel over time and not all immediately when patch 3.3.0 goes live.
The first section that opens will include the Lord Marrowgar, Lady Deathwhisper, Icecrown Gunship Battle, and Deathbringer Saurfang encounters. "Several weeks" until the next encounters become available.
Players may not attempt any Heroic versions of 10 player encounters until they have defeated the Lich King in a 10 player raid. The same goes for 25 mans. This means no heroic Icecrown until after the Lich King is defeated, which will apparently be, at the earliest, in a couple of months.
The four most difficult bosses have a limited number of attempts on them each week. Once those attempts are gone, you have to wait another week to try again.
There will be no explicit rewards for defeating the Lich King with a specific number of attempts remaining as there was with Trial of the Grand Crusader. There will also not be an achievement to complete Icecrown Citadel without being defeated by a boss encounter, or letting a raid member die. (i.e. A Tribute to Insanity).
In the weeks and months after all twelve encounters are unlocked, additional attempts against the final four boss encounters become available.
Over time, after all bosses are defeated, players will begin to get a buff, making defeating bosses easier.