Direct from the mouth of Ghostcrawler:
The nerf to armor pen in 3.2.2 is intentional. Compared to the recent buff where we increased the value of armor rating to 125%, this nerf would take it back down to 110%. While we are still evaluating the effects of this change in the 3.2.2 build, we did want to let you know of the possibility in case you were about to spend a lot on armor pen gems.
In fact, this was really the point. Several melee specs (and Marksman hunters) had begun to focus on armor pen at the expense of all other stats. Gear without armor pen was being passed over and gem sockets were increasingly being filled with just this one stat. While every spec has stats that are more valuable than others, this one felt like it was starting to trump everything. Not coincidentally, characters stacking lots of armor pen were starting to do more damage than their peers and more damage than we were comfortable with.
This change is largely for PvE reasons, though we won't cry at all if melee damage in PvP drops a little as a result.
We're letting you know now so that this doesn't feel like a stealth nerf, assuming it goes live. While you might disagree or be frustrated by the change (though I also suspect it won't come as a surprise to many players), we ask that you try and keep your response to something appropriate for these forums.
Great. Cool. Fantastic. This sort of stat stacking was always a risk given that the more you stacked APen the better the return was, coupled with some classes/specs still inflicting the majority of their damage as Physical. However, Armour Pen, previously tied for last place in Ret Melee stat priority at Tier8 gear levels (approaching the value of Crit Rating when you have oodles of it) is now back to being dead last. That makes thing very simple for us: generally avoid Armour Pen items unless they are an upgrade to the next gear Tier.
Just one thing though...
For Pete's Sake can you please stop putting it on all sorts of essential upgrades in Tier 8 and 9, upgrades that Blood/Unholy/Frost DK's, Warriors and Ret Paladins use. Stick it on Warrior and DK Tier sets, Blacksmithing crafted items, all that stuff. But stop using it on those shared items when for some (Warrior, Blood DK) it's a quality stat and others' it's dross.
This goes double for Weapons.
And triple for Ret Tier 9. Seriously, what idiot chose to stick Armour Penetration on our Tier 9 chest? Yes, it's still Best In Slot, but only Spellpower would make the use of item budget on this item more nonsensical for the spec.
And don't get any ideas from that for Tier 10.
Thank god this stupid stat is gone from items in Cataclysm.