Ok, I'm pretty sure my eyeballs are bleeding and my brain is hemorrhaging, but Armor Penetration has to be the most difficult stat to determine it's stat ranking.
Armor Penetration was buffed with the latest patch, which further complicates matters.
Armor Penetration gets stronger the more you stack whereas agility has to deal with diminishing returns for it's benefit to crit.
Here's were sh*t gets crazy.
Armor Penetration changeover where it rates higher than agility also is based on your overall stats, i.e. if you have high ArP, but low AP and Crit, then your ArP needed to make it more powerful than agility will be higher than someone who has higher AP and Crit.
Therefore the amount of ArP you need to stack to have it be valued higher than Agility is directly related to the overall AP and Crit you have meaning the better/closer you are to BiS, the more likely that ArP will surpass Agility as your main focus for gemming/gearing.
Basically from what I can tell, the lower your gear level is the less you will value ArP.
The higher your gear level gets, the lower the amount of ArP you will need for it to be of more value than Agility.
The hardest part will be factoring in the loss of AP/Crit from gemming for ArP in place of Agility and making sure that your gear has high enough AP/Crit for the changeover to have a positive affect on your dps vs a negative.
Also, at higher gear levels, with high amounts of ArP, the Shred idol overtakes the Rip idol from a pure DPS standpoint due to the high levels of AP/Crit and the amount of armor reduction possible on boss mobs.
Ok....so that's that. Hopefully that makes sense and we can somehow turn that into a coherent message to our droods. (Source #1) (Source #2)
EDIT: Keep in mind that there is still a lot of theorycraft involved so we will be modifying this as more info comes out obviously.
EDIT x2: OMG confusion will reign! Armor Penetration can put mobs below 0 armor and continue to increase damage past ability's base damage! O.o Edited, Apr 28th 2009 9:58am by Galenmoon