The patch is in preparation for the upcoming second expansion "Wrath of the Lich King", and will include many of the changes necessary for that expansion's release, including the adding of some new areas (Stormwind Harbor, Barbershops) and the removal of others (Naxxaramus, the Dalaran bubble) as well as a considerable number of changes for each of the nine current classes.
Wrath of the Lich King is due for release this November 13th.