Dark Iron Dwarves and Wildhammer Dwarves ARE ALREADY playable! When Cataclysm was released, they opened up the classes to multiple new races. Dwarves could never be mages, warlocks, or shamans before, but they said that the Wildhammers and Darkirons joined the Alliance, so it opened up those classes to people who made Dwarf characters. They also added new cosmetics to the Character Creator to reflect this, like black dark iron skin. They did the same thing with the Highbourne returning to the Nightelves, thus opening up Night Elf mages.
You people who think these "allied" races are a new feature, are probably just Panda babies who are swallowing Blizzard's claim that they're giving you something new.