You can get to the battery by removing the back of the case - 4 small screws. But it is a hassle (and I don't know where you would buy replacement batteries).
However, the battery is 2800mAh vs. the "standard" 1000 mAh of e-ink devices. I have an EB-1150 from about four years ago... different device, and it uses an LCD screen that's not as nice as the jetBook. The battery is still going strong. I guess time will tell?
EDIT: I figure in 3-5 years I'll be ready for a new device anyway.