STAGE 1 (100% to 65%)
Onyxia has 3 special attacks in this stage:

1. Fire Breath - cone-shaped AoE that does 3k+ damage
2. Knockback - throws you backwards
3. Tail AoE - does 300 dmg

What this means is that Onyxia must:
1) be attacking the MT only, and
2) be facing N the whole time
3) raid group split
into two.. one at NE, other at NW.

To achieve this...
MT - Positioning is key. Back must be facing the N wall at all times. Stand about 10 to 15 feets from the N wall.
DPS - Keep DPS low and steady. Mages do NOT over nuke. Rogues should shoot with guns, or feint if meleeing, and Hunters can Feign Death as soon as the cooldown is up.
Healers - All heals should goto the MT. Healing rotation is best (but we need to find out what's the best strategy here). Healers not in rotation can add to DPS.
Whelp groups - Need 2 of these. 1 on each side. They need to clear any whelps that hatched in all stages. Otherwise, attack Onyxia.

STAGE 2 (65% to 40%)
Onyxia flies up and leaves melee range. It has 2 attacks:

1. Fire Spit - does 3k+ damage to random, single targets
2. Deep breath - large wipe-causing AoE. Onyxia warns us with an emote "Onyxia takes in a deep breath"... this only happens when people are bunched up together. SOLUTION - keep space apart each player; if you see emote, run to the sides of the room (away from where the breath will be directed) and jump up on the ledge.

In addition, many whelps will spawn.
This means that: 1) Space yourselves apart
2) bandage yourself after you get hit by Fire Spit
3) focus on whelps in the intial stages of this stage until they are contained.

Specifically...
MT - Start collecting rage at 45%.
Melees - Taunt/etc whelps off casters.
Ranged DPS - unload all of your spells onto Onyxia, including DoTs and curses.
Healers - Keep whelp killers alive. Let victims of Fire Spit bandage themselves.
Whelp groups - Ignore Onyxia and concentrate solely on whelps.
Everybody - If you get hit by Fire Spit, bandage yourself immediately after.

STAGE 3 (40% to 0%)
Onyxia comes back down to the ground and periodically causes the lava cracks on the ground to shoot up causing damage to those nearby. In addition to the 3 attacks of stage 1, Onyxia also deploys an AoE fear and teleports the MT occasionally.
This means that this stage is pretty much like stage 1, except for the exploding lava, fear, and MT losing aggro periodically.

We need to:
1) Do what we did in stage 1
2) avoid lava cracks and stay in position up agains the wall on the ledge
3) reposition ourselves and stop attacking after avoiding fire breath when the MT loses aggro.

MT - After Onyxia lands, gain hate immediately and pull her back to the N position slowly, avoiding lava cracks.
DPS - Keep DPS low and steady. Mages do NOT over nuke. Rogues should shoot with guns, or feint if meleeing, and Hunters can Feign Death as soon as the cooldown is up. Also, avoid fire breath when MT loses aggro, and watch out for lava cracks.
Stop attacking and reposition immediately if you had to avoid fire breath or was feared.
Healers - All healers should be healing the MT at all costs.
Whelp groups - No whelps spawn in this stage, but if anybody aggros eggs, you must take care of them. Otherwise, attack Onyxia.

FINAL WORDS
1) Communication is really important. I liked Xorp's engage/disengage/etc yell macros. Very effective.
2) DPS must be kept low. This is the only way MT can maintain aggro for the whole fight. Use your aggro-losing abilities everytime it is available.
3) MT positioning is key. It comes with experience, but make sure your back is facing directly N. Also, by all means, make sure your HP is at its maximum potential.
4) Do not have your back facing the eggs.
5) Onyxia goes on a rage at 5%... doing much much more damage than before... persist to the end!
6) Druids innervate main healers and combat rezz dead priests.
7) We need Dwarf priests... Fear Ward for the win!
8) Rogues, bring lots and lots of ammunition.
9) We should try some kind of real-time audio communcation.

EDITED

10) After viewing the video Xorp linked, it seems that the first disengagement was at 70% (for regen/aggro management/buff purposes). Cease-fire lasted for about 40seconds. Also, it seems that the MT is the one calling engage/disengage. Anybody know why?
11) Just before Onyxia lands (between Stage 2 and 3), avoid the egg area to prevent from being feared and move towards orginal positions (NE/NW). This is the time when MT needs to reaggro and DPS needs to stop firing for 1min or so after Onyxia lands.
12) Mages/Warlocks do not over nuke. To maximize mana regen, it is wise to cast in bursts (10 nukes at a time, then rest to regen, etc)