Mojang fixed the issue, we are back online.
Quote from Mojang:
We are aware of the security issues involved with the Minecraft authorization servers and are currently working to fix it.
Right now the authorization servers have been taken offline and will be down until further notice. The Mojangstas are working hard to make sure we get everything back to perfect working order.
We’ll keep you updated as we have a more clear timeline. We really are sorry about this and are working as fast as we can!
Basically, there is a flaw with the minecraft login server which allows these "hackers" to login as anyone (even admins) on any smp server with mojang accounts. There were attempts on the WoM Public Server which were detected and blocked immediately, followed by a server save and safe shutdown. This happened a few hours before posting this message.
Once the security issues with minecraft have been fixed, we will be back online.
Further reading for technical minded people.