The website has been in this state since about 11:25 GMT. The first impression would be that their servers have been compromised and the hackers have put up the message. The US version of the site is still functional.
FreeBalance was probably the first one to spot this and have reported this on twitter already. We still don't have any official news about the reasons behind this strange message. We will update this post as soon as we get something.
[Update: 12:47 GMT] It seems that within a few minutes of us reporting this, the site is back up! We will still try and get the reason behind an hour long 'Hello World'.
[Update: 18:08 GMT] We haven't heard anything in this regards yet. So, I guess we will put this post to rest.
