Interesting observations from John Gruber on the iPad Pro and Pencil:

For capacitive (finger) touch, the iPad Pro samples at twice the rate of previous iPads — 120 times per second instead of 60. With the Pencil, though, the iPad Pro samples at 240 times per second. The way the Pencil works requires cooperation with the display, and so there’s no way this Pencil could be made to work with existing iPads. The Pencil is not iPad Pro-exclusive out of product marketing spite — it’s exclusive to the Pro because the two were engineered in coordination with each other. And if Apple had designed the Pencil differently, to allow it to work with existing iPads, there’s no way it could have had this level of accuracy, because the tip would have needed to be broader and capacitive. (The Pencil’s tip is not capacitive at all — it doesn’t register as a touch at all on any other iOS device.)

I think it’s quite possible the iPad ends up being a slow win for Apple unlike the quarter-after-quarter smash hit the iPhone is.