Wow-guides-noW

Icecrown will hold 3 wings for 5 man play, each wing is an instance of its own, and they will need to be finished in a pre defined order to be able to "unlock" one another.

The lore behind them: Your 5 man group is a small squad trying to penetrate the lich kings lines with the assistance of Jaina Proudmoore (Alliance) and Sylvanas Windrunner (Horde).

All that goes on while the lich king attention is distracted by the bigger army.

The 3 wings are:

The Forge of Souls
Serving as the first wing in this expansive dungeon, the Forge of Souls will quickly put players to the test of carving through the Scourge stronghold into deeper, more treacherous locations. Jaina will command Alliance forces, and Sylvanas will direct Horde forces. The goal is to ruin the twisted engines known as soul grinders found in this portion of the citadel, and then players can advance -- that is, if the Horde and Alliance forces can overcome the foes who confront them.

Bosses

    * Bronjahm, the Godfather of Souls: An instrument of reckoning, Bronjahm watches over the engines in the Forge of Souls. He must be killed if the soul grinders are to be destroyed.
    * The Devourer of Souls: As the chief operator of the engines found in this wing, the Devourer stands guard over the souls stolen by the Lich King.


Pit of Saron
Accessible only to those who have laid waste to the Forge of Souls' unholy operations, the Pit of Saron will bring Horde and Alliance forces deeper into the Lich King's domain. Players who venture here will immediately be confronted by the lord of this lair, Scourgelord Tyrannus. But defeating him will not be as easy as it seems. Before they can present a threat to Tyrannus, the adventurers, instructed by their leaders, will need to free enslaved allies who have been trapped by the Scourge. Until that happens, Tyrannus will leave all adversaries to his minions, workers of the citadel's mines. Perhaps the challenges here will lend clues as to the whereabouts of the Lich King's private chambers outside of the Frozen Throne, deep within the Halls of Reflection.

Bosses

    * Forgemaster Garfrost: A master of Scourge weaponry, the forgemaster hauls stocks of saronite ore and other precious materials to the cold forges where the mechanisms of death are born. With a host of rime weapons and exotic alloys at his disposal, it could get cold in here.
    * Krick and Ick: Zombies serve as mindless muscle in the Pit of Saron's mines, stockpiling metals for Forgemaster Garfrost, and Krick -- a devious leper gnome -- supervises the operations from atop Ick, Krick's ghastly means of transportation.
    * Scourgelord Tyrannus: Tyrannus is a terrible force who will no doubt demonstrate his powers to those brave enough to enter the Pit of Saron. The scourgelord must die if players hope to make their way into the third and final wing of this dungeon.

Halls of Reflection
With Jaina and Sylvanas leading the way, adventurers who make it as far as these frigid halls will quickly recognize the weapon that lies ahead: Frostmourne, the corruptive, legendary device of the Lich King himself. The Lich King's private chambers are within reach, although they may be the death of anyone who ventures there.

Bosses

    * Falric and Marwyn: Captains for Arthas Menethil in life, Scourge commanders for the Lich King in death, Falric and Marwyn will be summoned to the Halls of Reflection for one purpose: destroying all intruders.
    * The Lich King: Sylvanas, thirsty for vengeance against the corrupted prince who sentenced her to an existence as an undead monstrosity, and Jaina, eager to find a flicker of Arthas's soul locked somewhere within the Lich King, have brought their hand-picked allies to this final confrontation. Arthas's true power may only now be discovered. Is there any hope in this mission, or does only death await?

As more info is released about Cataclysm it gets clearer what causes (lore wise) the violent eruption that will tare the continents apart. It is also officially announced that outland and northrend will not be changed at all in this expansion.

Thus we can expect leveling from levels 60-80 to take relatively as much time as it takes to level them at the moment.

More about the cause to the Cataclysm:

To keep Azeroth safe from these threats, the titans imprisoned the Old Gods underground and banished the savage elementals to the titan-constructed Elemental Plane, a secure dominion where the elementals could dwell without harming Azeroth.

Four domains were created within this primordial realm to serve as ideal environments for each type of elemental. For ages since, the elementals of air, water, earth, and fire have engaged in endless strife with each other, waging titanic battles in the confines of their realm. One of the Elemental Plane's domains, a vast ocean known as the Abyssal Maw, is home to the water elementals. Legend says that Neptulon the Tidehunter, the powerful being who rules over the aquatic realm, possesses a potent weapon that allows its owner to control the very seas of Azeroth. Thus far, the artifact has remained secure inside the water elementals' domain... but that will soon change.

Within Deepholm, the domain of the earth elementals, the malefic Dragon Aspect Deathwing is on the verge of executing a catastrophic act: to burst forth from the Elemental Plane into Azeroth. Doing so will ravage the surface of the world and breach the barrier that separates Azeroth from the elementals' realm. Whether Neptulon and his subjects have entered into an alliance with Deathwing remains unknown. Regardless, if Deathwing's actions create an opening into the Abyssal Maw, nefarious and honorable factions alike will seek out the fabled weapon carried by Neptulon. Whoever secures the artifact will have control over the world's seas, a power that perhaps no one, not even members of the Horde or the Alliance, should possess.

More about outland and Northrend:

Just because Azeroth is being affected doesn't mean the threat of the Burning Legion isn't still there (for new toons to experience at least). If anything changes in this regard it would probably be a couple quests that send you there but I wouldn't expect Outland itself to change much since messing with 2 continents is already a lot of work. (Source) [...] You have to consider the changes with some form of leniency, while technically you could say that if the story progresses then we need to remove a bunch of the content from Outland and Northrend as those are over - but for numerous reasons, you can probably see why that would be a bad idea. This is the same type of thing that happens by just having a persistent world, just because one quest was completed where something was killed doesn't mean it vanishes from your sight forever. While a level 80 doesn't need to return to the locations of Outland and Northrend and can focus on the new stories - a new character hasn't helped defeat the villains in those areas so they still need to answer the call and fight them off (or in the terms people are tossing around, they need to level)..