So Blizzard has now created another... uh... thingy. These "Battle.net" accounts are supposed to link all Blizzard Entertainment game accounts into one account name and password. Which means all WoW accounts, all Diablo accounts, all Starcraft accounts, etc. are being merged into one account.
Pros: Only have to remember one account name and password.
Con: Gonna suck hell if you get hacked.

Read more at their FAQ page http://us.battle.net/faq/index.html