Update: The problem has been fixed. Carry on!
Mojang have turned off the authentication servers until an exploit in the system can be fixed. This means you can log in to the game but not join servers.
You can read more about this here:
Our server is not offline, but you will not be able to join our server or any other Minecraft servers.
Ungh… I hope they fix it quick
According to mojang and reddit posts, it is back up.
This site can help tell if the server is down or the problem lies on mojang’s side. http://xpaw.ru/mcstatus/
<blockquote>UPDATE: 8.41am BST: Mojang have pulled down the session server. This should stop the issue while a proper fix is being worked on!
UPDATE: 8.52am BST: mojang blog post
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 11:04am BST: session server back up. waiting for official confirmation from mojang that it's fixed
UPDATE 11.08am BST: xlson reports it is no longer possible to log in as someone else
UPDATE 11.14am BST: I've had independent confirmation from 3 users that the issue is now fixed. Thanks mojang! :D</blockquote>