As far as I know, their isn't a cap for ArP right now. It's a mystery. And Blizz isn't saying much 'truthful' information about it.
(if you read in the general wow forum
here you'll find more interesting info about ArP.)
To double post a few things and to just make it easier, look
here for elitist jerks information.
'Negative Armor' Values apparently DO exist. Check out
Nomepunter's stats. (yes that's RP's warrior).