NB: The Cataclysm strategies are based on a mixture of beta reports and may or may not bear any resemblance to the fight on Live.
If you spot major differences, post them in the comments and I can update the strats. I’ll be updating these myself as my guild begins raiding in Cataclysm.

Tanking: 2 tanks for Phase 1. The tanks on Cho’gall will need to swap due to the stacking Cho’s Blast and Gall’s Blast debuffs on his primary aggro target, which increase Physical and Shadow damage taken respectively by 20% per stack for 45s.
One tank will need to pick up the Corrupting Adherents (Faceless Ones) and take them away from the raid to die.

Healing: Since DPS is the deciding factor in this fight, due to the soft-enrage add-spawning mechanic, healing is likely to be squeezed.

Positioning: The raid should be spread out around Cho’gall, but not too far apart so that mind controlled players can be quickly interrupted.
A free space to one side should be left for the adds to be taken to die.

Buffs: Shadow and Fire resistance are valuable.

Key Debuffs: Cho’s Blast and Gall’s Blast – Stacking debuffs on the tank, causing them to take increased Physical and Shadow damage.
Conversion – Players mind-controlled with Conversion will worship Cho’gall, buffing him, until interrupted.
Corruption: Accelerated – Gained at 25 Corrupted Blood, this Magic debuff rapidly increases the target’s rate of Corruption until dispelled, so needs to be dealt with immediately.

HP: Normal: 101,000,000 (33,500,000 on 10-man)
Heroic: 146,000,000 (47,000,000 on 10-man)

Enrage: Unknown

Bloodlust: Phase 2 (below 25% HP) is a step up in terms of damage dealt and healing required, so it’s probably best to save Bloodlust for this phase.

This fight has two phases: 100%-25% and 25%-0%. It is also complicated by every raid member having a Corrupted Blood bar which fills as you are affected by various abilities throughout the fight. (See the table at the end for a full list) Corrupted Blood increases the damage you take from Corruption of the Old God by 2% per stack, stacking to 100.

At Corrupted Blood levels of 25%, 50%, 75% and 100%, extra things happen:

• 25% – You gain Corruption: Accelerated, a dispellable Magic debuff causing you to gain two stacks of Corrupted Blood per second for 60 seconds. This needs to be dispelled as soon as possible.
• 50% – You gain Corruption: Sickness, causing you to vomit, dealing 40,000 Shadow damage to players in a 5-yard frontal cone, adding to their stacks of Corrupted Blood. [Conflicting reports on whether this persists or fires once]. You should ensure that no one else will be affected by this.
• 75% – You gain Corruption: Malformation, which causes a Malformation to grow out of your back and shoot Shadow Bolts at nearby players.
• 100% – You gain Corruption: Absolute, transforming you into a Faceless One. You receive no healing but deal 100% extra damage and all spells are instant-cast. This is a last burst before you die from lack of heals.

The key then is to minimise your exposure to the effects that raise your Corruption level wherever possible.

Phase 1 – 100%-25%

The big concern in this phase is dealing as quickly as possible with the adds which spawn, large and small.
Periodically Cho’gall will summon a Corrupting Adherent, which needs to be killed as quickly as possible, since these will be the main source of Corrupted Blood in the fight. A tank should pick them up and tank them under the boss (to maximise damage to both) until they reach ~10% HP. Once an add gets low, the tank should drag it out to one side of the raid for it to be finished off by the ranged DPS. When it dies, it will leave a patch of Spilled Blood of the Old God on the ground which deals 15,000 Shadow damage per second to players within it, applying 5 Corrupted Blood per tick. You should not be standing in this.
Each additional Adherent should be moved to a point as close as possible to a previous pool of Spilled Blood.

30 seconds after the Adherent is spawned, Cho’gall will cast Fester Blood, which causes each patch of Spilled Blood of the Old God to spawn 5 Congealed Blood of the Old God adds with 300,000 HP (75,000 on 10-man). These adds are untankable and cause 2 Corrupted Blood with each melee swing.
They can and should be rooted and slowed and killed as quickly as possible before they reach the raid. If one is targeting you, move away from it.

If an Adherent has not been killed in time, it will gain Festering Blood, dealing 50,000 Shadow damage per 2 seconds to the entire raid. This is pretty much a raid wipe unless you kill the add it very quickly afterwards, so the Adherents really need to die in time.

The Corrupting Adherent also has a few extra abilities to worry about:

• Depravity – Deals 75,000 Shadow damage to the entire raid and adds 10 stacks of Corrupted Blood. This is a 1.5s cast and must be interrupted or Corrupted Blood will quickly get out of hand. A dedicated interrupter or two assigned to the Adherent may be helpful.
• Sprayed Corruption – Deals 55,000 Shadow damage in a frontal cone, applying 5 stacks of Corrupted Blood per 2 seconds. The tank should keep the Adherent faced away from the raid at all times and may be able to avoid this ability as well.
• Corrupting Crash – Fires a shadow missile at a random target, dealing 75,000 Shadow damage and adding 10 stacks of Corrupted Blood to anyone within 8 yards of the landing site. You should avoid this at all costs.

Besides the adds, Cho’gall himself has some interesting abilities as well.

First, every 25 seconds will alternate between casting Flame’s Orders, which adds 20,000 Fire damage to each of his melee swings and spawns fire patches, or Shadow’s Orders, which adds a raid-wide Shadow damage AoE to each of Cho’gall’s melee swings.

He’ll frequently cast Conversion on 3 raid members, mind controlling them and forcing them to worship him (channelled). If not interrupted, worshipping gives Cho’gall a stack of Twisted Devotion every 3 seconds, increasing his damage done by 10% for 20 seconds (stacking).
Everyone who can interrupt or stun should make this their top priority.

Finally, he will cast Fury of Cho’gall on his primary aggro target, which deals 30,000 Physical and 30,000 Shadow damage and stacks the Cho’s Blast and Gall’s Blast debuffs, increasing Physical and Shadow damage taken by 20% per stack for 45s. This means tanks will need to swap to prevent this debuff getting too high.

Phase 2 – 25%-0%

At 25% HP, Cho’gall becomes completely corrupted by the Old God, gaining the Corruption of the Old God buff, adding a 5,000 Shadow damage per 2 seconds raid-wide AoE and adding a stack of Corrupted Blood per tick.

He also summons 10 (4 on 10-man) Darkened Creations around the room with 166,000 HP (10-man and 25-man). These cast Debilitating Beam on random raid members, dealing 5,000 Shadow damage per second to them, adding 1 stack of Corrupted Blood per second, and reducing healing and damage done by 75% while the channel is active. The top priority is interrupting and killing these as quickly as possible before nuking Cho’gall down.

He will continue to cast Fury of Cho’gall in this phase.

Sources of Corrupted Blood