DuaneB:
I assume that the Arduino team are working on porting the existing servo library ?
I've pushed a first porting of the Servo library (the one distributed with Arduino IDE) for the Due, you can find it on the latest revision of the ide-1.5.x branch.
I did some rapid tests, and it seems to work.
DuaneB:
and so there would be nothing to gain by porting the following library other than the PPM Reading ?
Please do it, porting your 4017 library to the Due would be great! The Servo library "consume" 1 Timer every 12 servo connected, porting your library allows us to keep Timers for other tasks.