Things that need to be controlled have to be built to be controlled, and voice commands have always been a tricky thing. From what I have read, it is possible to use a Siri interface to actuate commands, but it takes a properly programmed computer between the iPhone and the internet to process the commands... and if it is anything like Siri, it won't be quick.
I was in Crestron training a number of years ago and they had been talking about voice actuated commands, but they basically hadn't been able to go anywhere with it. That was over half a decade ago, so maybe some things have changed, but I'm not sure if any control system supports native voice commands right now.
The flip side of all of this is the home itself.
TV - Do you have controllable displays? IR? RS-232? Ethernet? Just because something has an Ethernet port doesn't mean that it can be controlled over that port, so the manufacturer must have built the commands into the Ethernet design, and you will have to do work on controlling them that way if it is the goal.
Sound systems, like Denon's CI series, or Aventage from Yamaha, all have at least serial control, most work with Ethernet control. But cheaper components don't work that way.
Lights? Lights are worse, but you have to buy controllable lights - it's a very pricey endeavor.
Video game consoles, from what I've seen, suck. They are very proprietary systems, but I would think the X-Box would be more controllable than other systems like the PS3.
It would not require a lot of 'house' construction, but it would be a fun project if you have nothing else going on. You will really need to delve into the network control options, and network in-room options which are available and be prepared to buy a lot of gear which can be controlled.
I use a Crestron system in my home, and I wired the heck out of my home because I'm gonna be here for 20 years. Got a thread going on it which will be expanded as my system does...
http://forums.audioholics.com/forums/electronic-house-showcase-ce-pro-installation-center/83272-so-here-we-go-24-v-zones-w-10-sources.html
Controlling game system, lights, fireplace, surround sound, distributed audio, distributed video and the rest, but it's all centrally located, and I'm only using one control system. With a system that's split across several rooms, then you will either need a lot of Ethernet controlled gear, or you will need multiple control points in various rooms.
The voice side though... that's going to be the tricky part I think.
