Ghostcrawler has announced that armor penetration (ArP) is being nerfed in the upcoming Patch 3.2.2. It's an early notice to players who plan on investing in armor penetration gems. Many raiding melee players (along with Marksman hunters apparently) have begun focusing exclusively on armor penetration without caring about other stats. While the mechanics of ArP can be complicated to understand, the basic understanding is the more ArP you have the more damage you deal. Heck, a Feral Druid in my guild has been known to pass on upgrades here and there primarily because it lacked ArP.
Again, this is mainly a nerf to PvE raiding but it could end up affecting PvP slightly as well. It's not expected to completely rock the arena world or anything. The worst thing that could happen is Priests (or other soft targets) like myself will survive a few seconds longer against melee heavy teams, or people like Rogues have it worse against Plate. I guess from my perspective it could be viewed as a buff!
Note that in Cataclysm, armor penetration will be removed as a stat as it has been deemed confusing in regards to its use.