I believe they're adding a nickname system for replacement of the whole RealID thing for gamer friends that you don't know IRL. I think they called it BattleTag, read the preview a little while ago but here's what I remember:
battle.net account - account all your games are tied to, login for WoW, SCII, D3 etc.
RealID - aimed at people you know, handles cross comms between games - you add them once and then you see/can chat with them in other games, lets you see friends of friends too.
BattleTag - hand out to guildies and random gamers who you don't hate, to communicate without them knowing your real name, email etc. Should have been added with RealID communication stuff, still in testing I think.
Some things might be a bit sketchy in explanation but that's what the bits are I think.