I agree, Samer. This would have to be reasonably rock solid. A commercial example of this is:
http://www.rainbird.com/landscape/products/controllers/ETmanager.htm
I will look around to see if I can identify some solid algorithms that would take into account weather data (e.g. wind speed, humidity, temperature, rainfall, etc) that spits out a reasonable evapotranspiration value that might be easily used for this.
As for killing someone’s yard while on vacation, there are several issues here. First, if the hardware goes on the fritz in an infinite loop, this could lead to uncontrolled open valves and flood something, which would be bad. I would think that coming home to some dead plants would be the better alternative than a flooded neighborhood or thousand dollar water bill. Even so, this is a hardware problem that could be addressed with other hardware, but beyond the scope of our discussion.