We are aware of the security issues (people using others' usernames NOT any leak of passwords or personal information) involved with the Minecraft authorization servers and are currently working to fix it.
Right now the authorization servers have been taken offline and will be down until further notice. The Mojangstas are working hard to make sure we get everything back to perfect working order.
We'll keep you updated as we have a more clear timeline. We really are sorry about this and are working as fast as we can!
Happy Sunday :)
UPDATE: Woohoo! Things are back up and running perfectly! Thank you all for being patient while things were fixed. Also major props to Grum, Dinnerbone, and Leo who were out of bed and in to action in the blink of an eye!