Update: So, I unplugged the N800 from the charger and turned it off using the power button. I pulled the battery, left it out for 10 minutes, then put it back in. Using the power button, I turned it on, and this time got the blue line running across the bottom, etc. Now the battery indicator is back in the upper right corner, and it shows 1 day usage left.

I've currently got it plugged into the charger, where it seems to be doing its charging thing OK. I'm stumped as to why it turned itself off earlier today, and why the battery indicator went *poof*. (And of course, now I'm fretting that it's about to brick itself just like my first N800 did.)

Maybe this disappearing icon is of no significance, but I figured I might as well throw it out there in case it means something to somebody smarter than I am.