Do you mean FROST trap, the area effect slowing trap or FREEZE trap the ice cube? Some folks confuse these two.
If you mean Frost trap, I believe only the trigger person can totally 'resist' the trap so as not to trip it completely. If it trips, it's my understanding anyone remaining or running through the area will always be checking to see if they are affected by it. I have seen folks get nailed by it ¾ of the way through it. I don't think the mechanics of this trap have changed much. Someone else can verify or dispell my memory on that. I haven't used it in a long time. You don't really need it levelling.
If you mean Freeze trap, then it's always been of dubious use in PvP. For Arena, I've only had moderate success on Lock pets, maybe it could be used against DK pets but I'm not sure. Against a BM hunter pet it likely isn't very useful. It has far more utility in BG's than Arena's. In BG's I've had a lot of success with it but you need to be cagey and clever with them and catch the player off guard. It will give you that few seconds to set up and blaze off at a player, ususally enough to kill them before they can intercept you. In Arena, forget it, you have other things you should be doing.