On the N900 the display property "Display stays lit when charging" gives you exactly what it promises: as long as the device is charging, the display stays lit. However, if the device thinks its battery is already full, the display switches off (in the normal backlight time-out) even when a charger is connected (there seems to be some minor strangeness here, as a dedicated charger and a computer USB cable seem to behave slightly differently).

I believe most people are actually expecting an option "Display stays lit when hooked to charger"?