It seems that minecraft.net, Minecraft’s authentication service, and multiplayer sessions service are all down (source: help.mojang.com). You can check in here and we will post when these services are back up.
UPDATE: It seems that the authentication servers are fixed, and multiplayer is possible. However, minecraft.net still returns down (maybe a CSS/JS issue?)
UPDATE: All services are back up! Get back to playing!
It looks like it’s been down for at least an hour. What do you think? Minecraft 1.8 release? Or just downtime? (Maybe hackers :P)