I actually find the scroll motion to be very easy to perform using my index finger. However, I do agree there are issues to consider when it comes to usability. You definitely want to prevent unintended gestures and clicks/taps. So for one thing, the back touch doesn't necessarily have to be "on" all the time. It may be a mode that you enter under certain circumstances. And perhaps more advanced gesturing algorithms are needed to filter out intended from undintended gestures - maybe giving more weight to the size of the touch area, so that you palm resting on the back doesn't register as an event.

I don't pretend to have the answers to all these issues, but something tells me that smartphones can do much better than the current state of the art when it comes to game controls, to pointing at precise location, etc. And it goes without saying that this won't work with the existing mobile OSs as-is.

My hand is in contact with the touchpad on my laptop almost constantly, but this rarely causes any unintended interaction.

Same although it causes me problems frequently because I also use a mouse. In fact sometimes when typing I'll every so gently brush the pad and jump somewhere else and start overwriting things.

I had to turn off touch to click on my MacBook Air because I often brush the pad with the heel of my hand while typing, causing the OS to refocus on wherever the cursor happened to be.

Isn't it off by default?

I like the physical click way better, in any case.

I'm not sure what you're getting at. Your laptop is several times bigger than your phone so it's not as big as an issue. We are talking about a device that's smaller than your hand.

hes getting at the fact that laptops are trained to ignore input from your palm and only only respond to your fingers... a smartphone could also ignore certain inputs that are obviously not meant to be used.