Here on the first full release day of Blizzard's Diablo III live on Battle.NET for the whole world to play, the dreaded "Servers are down" message has been seen by thousands. Blizzard has acknowledged that this error is popping up across the planet and they have let the world know that a "quick fix" is in the works. You might already know that and are at this point only looking for some solace in any new news on when you'll be able to play again - unfortunately that news is only known by Blizzard officials themselves.
What is known is what glitch actually set this whole firestorm off. The most innocuous of things, a trade, a pushing of items is to blame. To make the error happen for yourself, and we do not recommend this if you're still able to play online at the moment, is to create a Demon Hunter and give a Templar follower a shield. Simple as that.
Once you do that strangely common set of actions, you'll be booted from the Battle.NET servers and will not be able to log back on until Blizzard sends out a software fix. This sad news is also popping up as Error 3006, the saddest of messages a Diablo fan can get after waiting more than 10 years since the last installment of the Diablo online RPG series. Now we must wait - oh the pain!
UPDATE: Servers are offline for the time being - hopefully not for long!
"Starting at approximately 2:15 p.m. PDT, we will be bringing Diablo III servers in the Americas offline for urgent maintenance to address several issues that are impacting the game. We anticipate that maintenance will last for about an hour and that servers will be available at approximately 3:30 p.m. PDT. We will provide further updates as they are necessary. Thank you for your patience." - Blizzard Forums