Update: The game servers are back up and we believe that problems described in this post have been fixed.
Old news post: We're getting absolutely crushed under the number of players trying out Path of Exile, and this has manifested as a server crash that has been occurring hourly today. In addition, there is a very rare bug that caused some players to log in to accounts from other players. The scope of this bug is very limited (and in almost all cases, the players logged out without disturbing anything). Fixes for both are being worked on at the moment. Because the game experience is compromised and people are disconnected due to the crashes, we have turned the game servers off until they can be brought up in a more stable state. I will update this post as we go - it shouldn't be too long! Thanks again for your patience, and sorry about these launch issues. It's amazing to see such high demand for the game.
Edit: The bug where players log in to the wrong account only affected people queued on a login server where that login server had also crashed. We have a fix for this that we are implementing and will test/build/deploy soon.
Edit2: For those interested, player concurrency hit 69850 players before it was high enough to take down the realm. Also, the current server problems do not appear to be to do with the DDOS we endured on the weekend. These are scaling problems with our servers and software, so we can fix them.
Edit3: Jonathan has made a serverside patch for the accessing-wrong-account bug that we are manually applying to the realm at the moment. Thankfully the number of people affected looks small.
Edit4: Servers are up and we're keeping a very close eye on their stability.
Edit5: We've found what is causing the realm crashes and are preparing a fix.
Edit6: The crash fix has been deployed and it shouldn't crash any more. I hope.
I'd like to take a moment to thank everyone for their patience with these problems. We're making big progress each day towards handling more and more players, so it'll be very stable soon.