On the one side, I think that Apple has spent so much time and effort perfecting the home button that it would be a shame to now reposition in on the back. Which would require a lot of resigning.
On the other hand, if apple does get rid of the home button and allows for some sort of sensor embedded in the screen. This will allow them to reduce the overall footprint of their devices and greatly improving useability. I don't think this would happen though. They could potentially use the fingerprint scanner thats available on the new Macbook Pros. Not sure if this is smaller than the round home button but could make the bottom chin a fair bit smaller.
