To quote wowwiki....
"Armor penetration is a value appearing on many items, talents, and abilities that indicates the percentage of armor that will be ignored by an attack. ArP can be found as both Abilities, Talents like Mace Specialization or as passive upgrades found on equipment pieces. ArP is unique in that the more ArP you have, the greater the increase in damage becomes."
Something you'll want to look at is the "weight" of a stat. This is done most the time to see the value of each stat according to your spec.
PVE Mutilate MAEP
1 Agility = 1 MAEP
1 Strength = .55 MAEP
1 Stamina = .01 MAEP
1 Hit Rating = .83 MAEP
1 Crit Rating = .81 MAEP
1 Attack Power = .50 MAEP
1 Expertise Rating = .87 MAEP
1 Haste Rating = .64 MAEP
1 Armor Penetration Rating = .65 MAEP
1 Resilience = .001 MAEP
PVE Combat MAEP
1 Agility = 1 MAEP
1 Strength = .55 MAEP
1 Stamina = .01 MAEP
1 Hit Rating = .88 MAEP
1 Crit Rating = .84 MAEP
1 Attack Power = .50 MAEP
1 Expertise Rating = .82 MAEP
1 Haste Rating = .73 MAEP
1 Armor Penetration Rating = 1.00 MAEP
1 Resilience = .001 MAEP
I have listed above the 2 most popular specs (as far as I can tell). I know those numbers look confusing... but lemme see if I can explain how this works. It is all based off of Agility... so if you look at the weight on the PVE Mutilate spec (1st list) 1 point of agility is equal to .65 points of Armor Penetration. So by doing the math on a piece of armor you can tell how many "equivalency points" you will be able to get from each stat. Let's look at a piece of gear as an example.
To do the math on this you would break it down as such (using the PVE Combat spec)
54 agility x 1 = 54 EP (Equivalency points)
77 stamine x .01 = .77 EP
53 Expertise x .82 = 43.46 EP
132 attack power x .5 = 66 EP
45 Armor Penetration x 1 = 45 EP
Total EP = 209 total EP
You can do that kind of math on any piece of gear really... but the stat weights, depending on your spec show how valuable that stat is.
For instance (using the combat spec) you can see that your stat important order is
Agility/Armor Penetration
Hit
Crit
Expertise
Haste
Strength
Attack Power
(
note.... I would say that hit is the most important stat till you are at the hit cap, but I don't raid with a rogue so one of our high end rogues would be a better person to tell you if hit is more important)
Depending on what spec you have at 80 you can choose how to itemize your gear. But to make a long story short.. armor penetration is an excellent stat all around.. but depending on your spec will show you how important it really is.
Also, an excellent resource so you don't have to do all the math on every piece of gear like I did above is
http://shadowpanther.net/
It is a great resource and I used it a great deal when leveling my rogue.