I know very little about this from a personal standpoint, other than SEEING the same gizmo inside the back covers of all the 3 Paperwhites that I've so far had the opportunity (I may say the PLEASURE) of peering into. That said.....
On youtube there's a trardown-with-commentary clip at https://www.youtube.com/watch?v=phO4QkHW...
which mentions the same thing. The blogger there (EEVblog) has a distinct british accent, but I THINK that what HE says is, that it's some type of "RF ID tracking device." (RF ID I take to mean Radio Frequency Identification)
See the clip yourself and especially check from 5 mins 20 seconds to 5 mins 42 secs approx. It's very informative for the most part, though most of the extra space that he mentions at 7:28-7:50, with a suggestion that it be utilized by installing a bigger battery, is actually occupied by a transmitting module - probably the 3G receiver- in the units that I've seen.
I hope this helps.