Exactly, @Seek_Kolinahr. Even in non-time sensitive situations, BRB is usually some kid who is going AFK just for the hell of it and won’t be back for a couple of hours. Maybe it really is a sudden, short-term thing like a pizza guy or getting the cat off of the fridge, but most BRBs I see are never right back; average is about 20 minutes, and sometimes I wander off in-game and five hours later get a /tell that they just came back!
In other words, BRB is, at best, vague, and often abused enough that BRB is often read as “Screw you guys, I have better things to do!” unless used within a group that is personally familiar with you, like a guild or clan. And if you are in a vital role on a team action, like a healer on a boss fight or an outpost Battle, BRB is even ruder; you’re actually walking out on a job that you said you’d do. Go up to your employer at a random time during the day, say, “BRB”, and just wander off somewhere for a few hours. See how well that goes over.
Biobreak is somebody who has a definite and immediate need to go, but will be returning shortly. We know that you have a very valid reason for your absence, but also that you will be back in a reasonable amount of time.
Second Life, eh? That doesn’t even qualify as gaming. It’s a virtual world and nothing more. If Second Life is a game, the so is walking down the street. When Second Life has PvP, wild animals, and/or open warfare, along with win conditions, I might change my mind.