ABL... If that is the case, I'm not sure what exactly we can do, other than, for instance, forcably maintaining the brightness level using a timer... That isn't a good option though, because that'll keep the tablet from sleeping properly and would waste battery. I really wish there were brightness change events we could watch for or something, but the current interface to those settings is abysmal.
That said, I should note that I am unable to reproduce this with my N800. Have you disabled your N810's light-meter? That could theoretically be causing it.