Battle.net outlines a new BattleTags system they are implementing, offering a new way for players to identify themselves and communicate on Battle.net. They say this is "similar to the Real ID system," though they also note that they are not unique, so multiple players can all use the same BattleTag. They offer the ability to create a tag right now, a FAQ on how this all works, and this overview:

A BattleTag is a unified, player-chosen nickname that will identify you across all of Battle.net -- in Blizzard Entertainment games, on our official websites, and in our community forums. Similar to the Real ID system, BattleTags will ultimately give players on Battle.net a new way to manage public profiles, find and chat with friends they've met while playing, form groups, and stay connected across multiple Blizzard games.

While most of these features are coming soon, players who wish to pick a BattleTag now can do so via Battle.net Account Management. Note that BattleTags are not unique, meaning multiple players can choose the same BattleTag. For now, we’ll be testing some basic in-game functionality (including in-game chat and friends list support) in the next patch for the Diablo III beta test. Players are also now able to use a BattleTag when posting on the Diablo III community websites.

In the future, BattleTags will be integrated into other Blizzard games and services such as World of Warcraft® and StarCraft® II (and their related forums and websites), and we look forward to sharing more about all of the ways they’ll help you connect and play in the months ahead. Stay tuned to Blizzard's official community sites for further details.