Both my VW and BMW do it. My experience is that if the catalyst is hot and the engine speed is high enough, the ECU decides to shut down unnecesary idle fuel. Atleast in the BMW with an analog mpg gauge, I can see exactly when the shutoff happens. Assuming the cat is hot, if the RPMs are above about 1500 while coasting in gear, it usually takes about 2-3 seconds of normal idle flow but then shuts off. It keeps it off until the RPMs fall below about 1000 (idle is about 650). If I'm on a long hill and I have to keep the speed from increasing (either for traffic or to avoid speeding), I can sometimes do this for a long way. It really pushes up the average mpg.
It's an interesting game to play because it's often better to 'spend' the momentum I've already invested in, and coast farther with the clutch in letting the engine idle. In these cases I also consider how far I am likely to coast. If it's pretty far, sometimes it's better to speed up slightly (as in a pulse) and then glide at a higher speed. Since the idle fuel flow is constant, my higher speed in these cases turns into mpg numbers.
And then there are times when I need to slow down and surrender my precious momentum, such as for traffic, stop signs, lights, sharp corners, etc. Rather than just squander it on merely heating and wearing my brakes, I try to take advantage of the shutoff function. In these cases, vehicle speed is irrelevant and RPM is all important. I have to choose the right gear and then brake only when I finally have to stop.
Now if only there was a way I could derive the defining algorithim. Of course then I'd want to add automatic shutdown/bump starts, then automatic FAS and restarts when stopped, regenerative braking, not to mention aero, A/C and P/S mods, and... :P
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