Anonymous claims it broke into the PlayStation Network and has a 50 gigabyte database of email accounts and their passwords. 10 million accounts "are at risk," says the hacker involved. The passwords appear to be encrypted, and many of the accounts published in this document appear to be from northern Europe. This still would be a great time to double-check your password security.
Kotaku has contacted Sony for comment and will update as the situation progresses.
[Update] The list in question appears to be a copy of this pastebin posting from March 19.
[Update] Shane Bettenhausen, with business development at Sony Computer Entertainment America, says the claim is "totally fake."
[Update] Sony itself says the claim is "completely false."