Certainly very interesting and something that I have thought about myself.
Have you worked with voice recognition technologies before? From what I have read a lot of complexities come from picking up voice reliably and what microphones to use. This could be overcome somewhat by incorprating the telephony system, so that you dialed a certain extension to issue a command. Not as simple to use, but probably more reliable plus the bonus that you could conceivably control it from anywhere you can find a phone.
I think activating a scenario (assuming that you have the recognition part working) should be quite straightforware. You are essentially doing a messageSend. This could be done by creating a "voiceOrbiter" device or something.
Controlling the menus might be better approached by simulating a user interface device. The Orbiter can be controlled by keyboard, mouse, infrared remote etc. so I imagine this should be quite doable. Potentially you could even write a LIRC driver that works on voice???
Configuration should be a ble to be added to the standard configuration screens I guess.
Let us know how you get on. You might want to take a look at MisterHouse as I believe that has voice control already.