I've been PvPing straight for 4 days now and it seems that MS won't proc on plate. Against a rogue, shaman, hunter, mage, priest, lock, and druid I get procs all the time, but with warriors, paladins, death knights, and bear druids I get 1 proc a fight. I recently got the 4 piece bonus that gives a 20% extra chance, but still nothing.