As some of you may know or have experienced, you are getting the same error message as you did several weeks ago when there was an issue with authentication minecraft servers, if you have trouble remembering, visit this topic http://www.noobscraft.com/server-announcements/mojang-authentication-server-down/. You were not able to get into multiplayer servers, a very similar issue has occurred once again but this time with player security, this is a minecraft as a whole issue and not the server. This security issue is not one to be joked about, players were and still are able to log in as others, thus many servers reported seeing Notch log onto their servers. Mojang has done this to protect your accounts, they are working as quickly as possible. Please be patient while the issue is sorted and hopefully we will receive more information as time progresses. Sorry for any inconvenience.

For clarification, here is the official mojang topic:
http://www.mojang.com/2012/07/houston-we-have-a-problem/

Official Noobstown Announcement:
http://www.noobscraft.com/server-announcements/minecraft-net-downtime-(security-issues)/
Refere to the link above, topic locked.