A very good post on Armor Pen.
Vallen had a really nice post on Armor Pen everyone should read which touches on the upcoming ArP nerf as well as some good rules of thumb.
My personal opinion on ArP is not to gem for it or drive yourself crazy trying to get it. This includes getting 1 of the 2 big daddy ArP trinkets which requires either Hard Mode Thorim in 10 man (not that it is hard once you get it down) or 25 man Naxx (not hard but not fun either).
Without either of these 2 trinkets there really is little reason to stack ArP. I like that Vallen mentions to think about what is reasonable gear you can obtain. So many DPS models take for granted you are just going to fly through the hardest of content and get every piece your heart desires which couldn’t be further from the truth.
My current plan right now as you can see on my gear page is to obtain 2T9. Shoulders and Gloves for 60 Triumph Emblems is the current route I am taking. In RAWR this is about the same DPS increase as keeping 4T8 and going with the Duskstalker Shoulderpads. Nice thing is I maintain my Hit slightly above the Cap which I like.
I have decided that breaking 4T8 for 2T8/2T9 is probably the best idea so I can start gearing up from better non tier gear in ToC 25 normal. First change will most likely be swapping out either Chest or Legs pushing towards eventually giving up the 2T8 bonus altogether and sticking with 2T9 in the Helm and Hands positions somewhere down the line.
If nothing else making the change from 4T8 to 2T8/2T9 will allow me to make some live testing to see how they compare.