When two players open chat at the same time in a game of 0.8, all queued messages rapidly disappear (5-15 seconds). This includes any existing messages and all messages sent while the window is open. However, if you reload the client and open it when you are alone, all of the messages reappear. I don't know if this is a bug, but it is very tough to keep track of anything.
Also, it's neat that it updates rapidly and you can "chat" there like an instant messaging service.