I'll admit it: Armor Penetration has always confused me. I understand it moderately well - it makes your attacks act as if your opponent had less armor. The conversions from numbers to percentages, and the corresponding DPS increase, can be found at various places on the internet. But due to the fact that armor doesn't scale linearly, neither does ArP: going from 50% to 51% ArP helps you much more than going from 10% to 11%. This makes it pretty hard for me to get a good feel for.
As a consequence of the non-linear nature of ArP, getting very high amounts of it can lead to insanely boosted damage - as an unattainable example, I'm told that 300% ArP would mean thousands of times more damage. To keep ArP from getting out of control, then, Ghostcrawler has announced that they plan on capping ArP at 100%, meaning armor could no longer go negative (which it can now).
This was made with an eye towards future tiers; at the moment, it's impossible to get to 100% unless you have very specific gear and two trinkets proc at the same time. So this should have very little impact, if any, on current DPS. However, we may see a day in Icecrown when ArP, like Hit, is a completely worthless stat after you have enough of it. This strikes me as an awkward situation, but short of removing the stat entirely (which, sadly, I don't think they're going to do), I can't see a much better solution.