If you are a fellow Minecrafter, read this for a warning. If you don't play Minecraft, just read this post for fun.
A former server Moderator from Diamcraft has reported that there were several hacking's on Minecraft reported. I have been a victim myself. Numerous hackers were logging into a Notch's workers accounts, also, other peoples accounts too. One of the hacked accounts was purposely hacked to grief and not get caught. He was caught and quickly taken care of. Reddit has told each of the admin victims to shut down their server temporarily. Mojang then found out about the exploits and is working very quickly to solve the problem. As a former Moderator of a server, I highly suggest to change your password. There are still ways to login as someone else, but they are being taken care of.
UPDATE 11.14am BST: I've had independent confirmation from 3 users that the issue is now fixed. Thanks mojang!
UPDATE 11.08am BST: xlson reports it is no longer possible to log in as someone else
UPDATE 11:04am BST: session server back up. waiting for official confirmation from mojang that it's fixed
UPDATE 10.22am BST: xlson reckons it may be a few hours before its resolved and the session/login servers are put back online.
UPDATE: 8.52am BST: mojang blog post
UPDATE: 8.41am BST: Mojang have pulled down the session server. This should stop the issue while a proper fix is being worked on!