Because of flaws in the minecraft client someone has made a hack pisible to asume other peoples ID. Also flaws in the server security have alowed to login as and take on the ID of other players. As far as i know the minecraft login varification is down and the security server is down.
Here are a cuple of links explaining what has happend.
Notch posted on reddit:
Team AVO who first found out about the problem:
And Mojangs official post:
I'll be sure to keep you up to date, but for now it's best to stay offline. Mojang have said that they are working on the problem and should fix it in a fiew days. Don't worry the world is backed up.