The realms will be coming down for an extended maintenance on the morning of December 10 to apply patch 5.4.2. The major new features of this patch are the cross-realm raid browser and cross-realm mailing of account-bound items. Also added in this patch is the jet-powered Iron Skyreaver mount, the source of which remains a mystery.
You will find the cross-realm raid browser in the raid tab of your social screen by clicking the "Other Raids" button. Here you will be able to list yourself as looking for a group for Siege of Orgrimmar flexible raids or Pandaria world bosses. Cross-realm mailing works for account-bound items only and won't work for gold or other items. To mail something to a character of yours on another realm, simply send the mail in the following format: CharacterName-RealmName.
Hit the break for the latest version of the patch notes.