Problem is still… wait… WTF???
Ummm… Ladies and Gentlemen, the problem seems to have spontaneously disappeared. I was coming in here to post that I took astro’s suggestion and the problem was still present in the BIOS. I exited out of there (without changing anything) and started windows, opened the browser, and began to post this response. (That opening line of this post happened in real time, no joke).
(Incidentally I did check the event logs and didn’t see anything that would correspond to this problem).
Also, now when I take the AC cable out, Windows does again recognize it and the systray icon changes to a battery. 
I mean :):)
but still
Also, I did flash the BIOS with what I believe was the same version as was already on and I updated a couple other drivers from the Dell site, as per voltaire’s pre-previous post, but the BIOS check was after all this (with a reboot in between).
In any case I am relieved (if a bit unsettled).
Many thanks for everyone’s input!