care to describe a potential application?
Here's a simple one that popped up immediately: scoring a Pinewood Derby.
At the finish line, you set up 2 photo-interrupters on each lane, wired to an Arduino. It can then both tell who came first, and how fast each was going, and relay that to the scoreboard system over a noise-tolerant 2-wire connection, instead of trying to connect all the individual sensor signals. You could also put photo-interrupters at the starting line with another Arduino, and time the whole race (+- a little for communications latency).
Or make an Arduino-based outdoor weather station with only two wires going back to the indoor PC.
It's kind of like being able to design your own Dallas "1-wire" devices, but even better, because the power/data wire can be used to supply other devices like servos and relays.