It's actually the opposite. From my understanding the light sensor on the N810 controls the screen like this:
imagine the light sensor reads a value from 0 to 10 (0 = dark, 7 = normal light in a room, 9/10 = sunlight).
As the value travels from 0-8 the N810 brightens the backlight to try to keep the screen visible. Once the light sensor reads 9/10, the backlight is turned off so that the transflective screen operates (instead of using the backlight, transflective essentially means light from outside is reflected back by the LCD to produce the image; thus no backlight is required, but the display is only as bright as the light shinning on it... thus why it is only used in direct/bright sunlight instead of all of the time).
My brother has an 810 that I've been dying to see in bright outdoor conditions as he says it's great, doesn't even notice the difference (compared to my N800 which is damn near impossible to see in such situations), but it's been gray and cloudy for a long time here (good ol' Canadian Winter...)