I've been wondering the same thing myself. (I have a PRS-505 on order.)
However, given some of the troubleshooting reports I've been reading in these forums about the PRS-500 and the Cybook Gen3, I'm expecting the screen is going to be a problem much earlier than the battery will. The screen does not seem to be covered in the warranty even though (or because) it's the most expensive, fragile, and unproven component of the device.
I'll be quite happy if my Reader has a useful life of 18 months. Something better will be out by then and I'll be looking for a replacement at that point. In short, I've stopped worrying about the battery.