Good point. In my car, the revolution sensor is used by the TCM in the gearbox and the ASCD (cruise) but the ECU gets its speed data for the Driver Information system from the 4 ABS sensors.
I wonder if in the newer models, maybe they added a check of this revolution sensor to the ECU because they were having troubles with ABS lines being ripped out, or ABS gear failures when bearings started to collapse? Maybe they cross-linked the checks with the ASCD? The speed data is reported via the CAN, so it won't be a loose wire or anything (because the CAN is obviously still getting other data from the gearbox). If it's the rev sensor, it'll be the sensor itself. Very unusual failure if it is.
So that adds a question: does the error only happen when you've activated cruise control, or at any time?