Issues with authentication
It looks like Blizzard are having some issues with certain battle groups and all authentication services this afternoon.
'The Battlegroup outages are now causing authentication issues across all realms. If you have trouble logging in, please try later.'
They have found the problem but given a 2-3 hour fix time. If you are online, try to avoid logging off unless you want to spend ages trying to log back in. If you are trying to log in, don't panic, it will all be fixed in a few hours.