Does anyone know of a house-plant watering system that uses a moisture
sensor to water plants via an electronic valve?

None of the "automatic" watering systems out there work. They all seem
to be either passive wick or timer-based systems that keep the soil so
moist that root rot always occurs.

What I'd like to see is a Thirsty Light moisture sensor (http://
www.thirstylight.com/) cobbled together with the Orbit electronic
watering kit (http://www.orbitonline.com/products/Timers/
01/04/01/1195/). The trick will be interfacing the sensor with the
watering kit. The watering kit is actually meant for outdoor use, but
it looks like the cheapest source of electronic water valves.