This posted by Mojang:
Houston We have a Problem…
by Lydia Winters on July 15, 2012
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!