Thomas Ricker and Jacob Kastrenakes, The Verge:
Each new Wi-Fi device also shares one common trait: an Apple TV is required if you want to control them with Siri while away from home.
So, while commands like "Siri, turn off the lights in the living room" will always work while connected to your home Wi-Fi network, they won’t from the airport unless you have an Apple TV. But that’s it - you can still switch off the lights with an app, no Apple TV required. This behavior has been confirmed by a source close to HomeKit’s development as well as two launch partners who wish to remain anonymous. Only third-generation or later Apple TVs running software 7.0 or later will support HomeKit.
While it appears that Apple TV won't be a full-blown home automation hub, Christopher Breen almost nailed its role back in June with his article 'Apple's HomeKit hub may already be in your house'.