Not the first time we've seen something like this: Nakatoir of the EU community team posted this account security alert after Riot Games' EU branch warned its users that hackers "gained access to certain personal player data contained in certain EU West and EU Nordic & East databases." This information included email addresses and encrypted account passwords, and more than half of the passwords were considered simple and at risk of being cracked.
Blizzard issues its security alert because many players who play various Blizzard games like WoW and Diablo III or StarCraft II also play League of Legends; therefore, if they use the same email address for Battle.net as League of Legends or the same passwords, those Battle.net accounts may also be at risk.
This is not an announcement that Blizzard itself has been hacked, mind you. It's simply a precaution based on the habits of players of many games to use the same passwords and login information for multiple accounts. If you're not a League of Legends player in the affected EU regions, there's no way for this to affect you.
The full announcement is after the break.