According to Computerworld, more than 10,000 Hotmail user names and passwords were leaked by an anonymous user on pastebin.com. Domains of @hotmail.com, @msn.com and @live.com were affected. While the post has since been taken down, Neowin.net says it "can confirm the accounts are genuine and most appear to be based in Europe."
It's not clear how the usernames and passwords were obtained, but Neowin speculates that they were the result of either a hack of Hotmail or a massive phishing attack. BBC News reports that Microsoft is "investigating the situation."