After some careful thought, it seemed like as good a time as any to say goodbye to that most controversial of stats, armor penetration. As of this writing, it's not even going to be baked into Battle Stance any longer once Cataclysm hits, so we can essentially call an end to warriors' bypassing or ignoring armor the way other classes do with magic damage. Like it or hate it, all warrior damage save bleeds will now be mitigated by armor. Yes, even Thunder Clap.
So what, you may ask? What's the big deal about armor penetration, anyway, and who cares about its not being around anymore in Cataclysm? Well, the short answer is, it lets our damage penetrate armor better (that is to say, it reduces the amount by which armor mitigates our damage) and warriors (and feral druids, and some rogues) care because our damage is overwhelmingly physical and thus reduced by armor.
The long, strange trip of armor penetration in this expansion started at launch with ArP being a somewhat undesirable statistic. It had been converted to a rating following its rollicking high in The Burning Crusade, due to the way it became such a devastating staple of warriors in PvP.