Blizzard introduce BattleTags
Blizzard are introducing a new identity feature that players can use to represent themselves across the realms of Blizzard games and community boards. BattleTags will first be available within the Diablo III beta and later it will be implemented into World of WarCraft and StarCraft II alike.
BattleTags are and can be created as of now through your battle.net account (simply click this link) but currently only Diablo III beta testers will be able to use them. What the tags really are is a combination between a nickname chosen by you and a four-digit number assigned by battle.net.
It is important to know that BattleTags do not replace Real ID in any way - it will be just another way to find people on the battle.net and add them to your friend list. Also, it is not compulsory to get your BattleTag right away, unless you want to log in the Diablo III beta or post on the game's community boards. For World of WarCraft and StarCraft 2, the BattleTags will be implemented "in the near future".
Battle.net - Source