Wearable Computers at Regular
I wouldn't accept them in a Magic game, period. That's far beyond what a mobile phone can do, and I think it doesn't create a fun environment for anyone.
As a player, I'm usually pretty annoyed if my opponent keeps on looking at his phone (apart from short and important calls, I'm not a jerk), even at Regular REL. Or if he listens to music or something. I'm here to play Magic and that's a disrespectful attitude. In CompREL, I'd call a judge. In Regular REL, I'm usually the judge myself. In both cases, my opponent will stop using the phone or playing Magic, he chooses :)