I'm repeating myself but, once again:
#1 You will not accidentally interact with objects while scrolling.
Ever been taken away from a page that took forever to load because it thought you clicked a link when you were trying to scroll and then spent twice as much time trying to get back only to have it happen AGAIN?!
#2 You scroll when what you are reading does not fit on the screen and your thumb will no longer obscure even what does fit on the screen (exacerbating the problem). Furthermore, you can't scroll while you read without repeatedly repositioning your finger at less than full page intervals (see problem #1 again).
This solves both problems that are the bane of all touch screen devices.
#1 - Android is worse about this than iPhone. I find that if I tap to scroll on on an iphone and I accidentally hit a link, I can either move my thumb to scroll and it will ignore the tapped link, or I can hold it long enough for the menu to pop up, in which case I can cancel. On Android, it seems to take these accidental clicks very seriously, activatingly the link after I scroll. Chrome is better about this that Browser was though.
#2 - I've never really had an issue with this. Yes, sometimes I'm only reading "half a screen" of text at a time, but it's never been a big deal. I fail to see how creating a scroll pad on the back would alleviate this problem, especially since you'd seemingly have several finger tips on the back of the device. You'd have to constantly reposition your fingers.
Have you tried to replicate this behavior on your phone just to see how it works? For me, it would basically just change how I hold my phone. Rather than my fingers resting on the back and holding the device up, Now, I have to "grip" it by putting my fingertips all the way around to the edge of the device and holding the opposite side with my thumb. Now my thumb isn't freed up to interact with the screen, and my index finger doesn't have a huge range of motion on the back of the device. Simple scrolling would seemingly be the end of what I could do in this position.
And there's no way I'd be able to interact with page elements, like links or forms, without repositioning the device in my hand to let my thumb navigate the page. The only solution to that, I would think, would be to replicate a mouse cursor that you'd move around with your index finger.
But then if you're replicating a mouse cursor, how do you scroll? Do you push the mouse cursor to the very top or bottom of the screen? Kind of takes away from the simple scrolling aspect.
Now we're thinking in desktop paradigms. Now we're at "How do we replicate desktop functionality" vs "How do we best navigate the device?"
Again, its a wonderful idea and kudos for trying to think outside the box, but this seems like a solution looking for a problem.