I thought about doing something similar with my thermostat, but I want to just get rid of the old thermostat and use an micro controller with a thermistor to control the furnace/ac, which would leave room for time-based controls too. I'm not sure what the voltage levels are on standard thermostat lines, but I'm sure it can be easily controlled with some transistors and/or relays.
I've had a few other ideas too, like having different temperature sensors for each room with servos controlling the vents, so that the room would only get heat/ac when it needed it, which means either more even heating/cooling or you could shut off the heat to rooms you aren't using.
I think it might be even better to use some type of rfid system so it knows when you leave.
