You misunderstood me. I was just saying the 500 point reduction was more than twice as effective as the 250 point reduction. NOT that the additional 250 point reduction gave twice the effect as the first 250 point reduction. I'm in perfect agreement that the scaling effects of stacking armor penetration is very marginal. But I just wanted to point out how it worked.
The DR formula is: Armor / (Armor+400+85*(Level+4.5*(Level-59)))
At level 70 that is Armor / (Armor + 70057.5)
500 armor is 0.70864188782198915777911632356589 % DR
250 armor is 0.35558084130427052590406428901611 % DR
0 armor is 0% DR
Assuming you're doing 1000 DPS before armor and a theoretical mob with 500 base armor.
At 0 armor reduction you're doing 992.91358112178010842220883676435 DPS
At 250 armor reduction you're doing 996.44419158695729474095935710984 DPS
At 500 armor reduction you're doing 1000 DPS.
Going from 250 to 500 armor reduction is a difference of 3.55580841304270525904064289016 DPS
Going from 0 to 250 armor reduction is a difference of 3.5306104651771863187505203455 DPS
Going from 0 to 500 armor reduction is a difference of 7.0864188782198915777911632356589
2*3.5306104651771863187505203455 = 7.061220930354372637501040691
7.0864188782198915777911632356589 > 7.061220930354372637501040691
So going from 0 to 500 armor reduction is more than twice as effective as going from 0 to 250 armor reduction.
But obviously, as you can see, the difference is minute. It is only 0.3% more. I would not spend time trying to stack armor reduction for some magical cumulative effect. It's tiny.
MAJOR EDIT: I may have made a mistake somewhere, as the poster above me has mentioned the constant for armor is around 10000. I remember it being 10,535 or something to that extent. The formula I used was from wowwiki,
Armor - WoWWiki - Your guide to the World of Warcraft and I believe it could be wrong. But as the above poster mentioned, there is some constant (k) where DR is evaluated as Armor/(Armor-k), so while the numbers may be off in reality, the concept they illustrate should still be valid.