On January 14, 2013 Oracle released an update for Java, the application used by the Chat Room application that by default causes access to be blocked.
In order to restore access to the chat after updating to Java 7 Update 51 (J7U51) you need to make a change to your settings.
NEW: We we have released a patch for Windows that will update the settings for you.
This patch can be be downloaded here
Manual instructions below