...Just to clarify, that would be a completely black screen with the backlight on, and it also depends on the resting state of the pixels, which I don't know for the new super LCD 3 technology (some LCD's have the resting state with the pixel off, some have it with the pixel on)
The actual shutter for the LCD use so little power you would never notice the difference though...
However it is completely the different for AMOLED screens, since each pixel produces its own light.
Here is a fun little site to look at, made by someone that has way too much time on their hands...
And here is a page from Nokia regarding AMOLED screens vs older TFT screens with a nice picture graph