Blizzard has set a deadline by which all World of Warcraft players will need to merge their WOW accounts with a new Battle.net account: November 11th in North America, and November 12th in Europe.
Battle.net provides a unified login for Blizzard games, including up to eight separate World of Warcraft accounts and future releases StarCraft II and Diablo III, as well as beta tests and the Blizzard Store.
In future, it will also unify chat, achievements and friends lists across those games, as well as allowing cross-server chat within WOW. However, Battle.net's most exciting features are reserved for, and will launch alongside, StarCraft II next year.
As incentive and thanks for merging their accounts, WOW players will receive an in-game penguin pet, including those that have already switched over. The penguins will be delivered later this week.