The granddaddy of the WoW-blogosphere, Big Red Kitty, has an article up today discussing the math surrounding the new "Ignore Armor" stat that was recently introduced on some pieces of gear in patch 2.3. Here it is: What's this "Ignore Armor" Stuff?

The examples that BRK provides assume an "Ignore Armor" value of 70, and look at target values of 6142 (his own armor) and 17801 (his main tank's armor). The results are:

for 6142 armor -

You've just reduced the amount of physical damage mitigated by armor from 36.78% to 36.51%. Congratulations!

and for 17801 armor -

You have now decreased the amount of physical damage Kel mitigates from 72.48% to 72.43%... Congratulations? Kinda.

Unfortunately, both of those numbers are utterly useless to a raider trying to make a gear decision without a bit more context. Our primary targets are level 73 bosses - not level 70 hunters or warriors. Thankfully the wonderful folks over at Elitist Jerks have deduced the armor values for many end game encounters:

KARAZHAN
Attumen the Huntsman: 41.5% -- 7500
Midnight: 43.5% -- 8200
Maiden of Virtue: 39.0% -- 6700
Julianne: 36.5% -- 6100
Romulo: 42.0% -- 7600
Strawman: 41.0% -- 7300
Roar: 31.0% -- 4700
The Curator: 36.5% -- 6100
Shade of Aran: 26.5% -- 3800
Kil'rek: 24.5% -- 3400
Terestian Illhoof: 39.0% -- 6700
Nightbane: 42.0% -- 7600
Netherspite: 34.0% -- 5500
Prince Malchezaar: 42.0% -- 7600

TEMPEST KEEP
Al'ar: 40.0% -- 7000
Void Reaver: 50.0% -- 10700
High Astromancer Solarian: 37.0% -- 6200
Kael'thas Sunstrider: 6200

SERPENTSHRINE CAVERN
Hydross the Unstable: 7700
The Lurker Below: 7700
Leotheras the Blind: 7700
Fathom-Lord Karathress 6200
Morogrim Tidewalker: 7700
Lady Vashj: 6200

Hyjal Summit:
Rage Winterchill: 6200
Anetheron: 6200
Kaz'rogal: 6200
Azgalor: 6200
Archimonde: 6200

Black Temple:
High Warlord Naj'entus: 7700
Supremus: 7700
Shade of Akama: 7700
Teron Gorefiend: 6200
Gurtogg Bloodboil: 7700
Reliquary of Souls:
- Essence of Suffering: 0
- Essence of Desire: 7700
- Essence of Anger: 7700
Mother Shahraz: 6200
Illidari Council:
- Gathios the Shatterer: 6200
Illidan Stormrage: 7700

OUTDOOR
Doomwalker: 42.0% -- 7600

Thus, BRK's first set of numbers (that of a level 70 hunter) are much more applicable to end game raiding. Whip out a calculator and run through his formulas if you are interested in the actual reduction values for whatever given amount of "Ignore Armor" that you are targeting. Bonus points if you post the results of your work on a blog somewhere. (Update: Someone took up the challenge - check it out!)

It is also worth recalling that survivability is impacted by armor in a linear fashion. Unless I'm totally out of the loop and raid bosses are unique in this respect, reducing an enemy's armor by X will reduce it's lifespan by a fixed amount, regardless of how much armor it actually has.

So what does this all mean to a feral Druid tank? Well, not much really. We should certainly not be going out of our way to acquire "Ignore Armor" gear, however if we happen across a piece that has decent benefits in terms of mitigation or survivability, then the Ignore Armor will bump our threat output a little bit, which never hurts.