Hmm. I posted just last night in the Power On/Off Enigma thread that my battery icon had mysteriously disappeared. What I did was take the battery out, wait 10 minutes, then put it back in; this made the icon reappear and I was able to recharge the battery.

If I read your post correctly, you did not take out your battery at first, but continued to use the N800 while the icon was missing. Since mine would not charge properly while the battery icon was missing, I'm guessing yours didn't, either, and it was probably not really charging last night. This is all pure guesswork on my part, based on what I observed with my N800 last night, but I think you might want to try swapping out with a charged battery if you have one, to try to get that battery icon to come back.