Aram is All Random All Mid and in the minds of most players denotes SR.
Arab is All Random All Bot and in the minds of most players denotes Dominion.
Proving Grounds has no other lanes, so calling it ARAM doesn't really make sense...plus it's too easily confused with SR's ARAM, which I'm quite sure people will continue to play.
So far i've just been using the following:
ARPG - All Random Proving Grounds
It's easy enough to say, but i'm curious what others are using...or if everyone is just misleadingly calling it ARAM? :/