As you could of probably guessed from the title, the minecraft login servers are currently offline. This is due to a number of security issues that are currently present with the authorization process (you logging in to a server) on Mojang's end.
You will still be able to login to minecraft to play single player ONLY but will not be able to play on multiplayer during this time.
There is currently no estimated time on when the authentication server will be brought back online, but be assured that Mojang are currently working on it to get everything back in working order.
So it could be anything from 5 minutes to 5 hours (let's hope just 5 minutes)
You can read the offical statement from Mojang here: http://www.mojang.com/2012/07/houston-we-have-a-problem/
and you can check on the minecraft server status here: http://xpaw.ru/mcstatus/
See you on the server soon :D
Mojang have stated that no personal details were exposed, it appears that they are fixing the session stealer exploit, read their tweet here: https://twitter.com/MojangTeam/status/224426440647651328
A big minecraft youtuber known as BebopVox has posted a video on the situation detailing how big the exploit is and what you can do to prevent your account from getting compromised: http://www.youtube.com/watch?v=US53lAyUN-8
There is a reddit post on the minecraft sub reddit with a timeline of what is happening and updates from Mojang on the issue: http://www.reddit.com/r/Minecraft/comments/wl0zy/psa_exploit_in_minecraf...
Github has a helpful article regarding the issue, explaining how this issue is caused: https://gist.github.com/3115176
If you have a migrated minecraft account (you login to minecraft through your e-mail adress), use this to check if you account is vulnerable or not: http://www.teamavolition.com/index.php?app=ccs&module=pages§ion=page...
MCBans have made an article on their website explaining what they have done throughout this exploit and what this means for the service and servers: http://forums.mcbans.com/topic/881-mcbans-actions-throughout-the-exploit/
The exploit has now been fixed and the authentication server is now back online. Thank you for your paitence.