Batten down the virtual hatches, Battle.net has been breached. Blizzard has posted notification of an illegal intrusion into its network during which e-mail addresses and encrypted passwords were accessed.
For players on North American servers, the intruders also accessed secret question answers and information relating to Authenticators, but Blizzard does not believe enough information was stolen to access accounts. The company also states that it does not have evidence suggesting real names, addresses, or credit card numbers were accessed.
You should change your password, especially if you’re a North American player, but don’t panic — it’s very, very unlikely that your plaintext password will be exposed. Still, change your password, and be wary of phishing.