Well I'm not sure where the 2 1/2 times diagonal of monitor comes from. Generally the figure I use is about 70 cm from the screen, but that's for regular (i.e. 15" to 19") monitors, which gives a ratio of about 1.6 or so; I've measured my distance and it really is that far away when I'm sitting comfortably at my desk. Is it like the distance where you can see the whole screen without having to move your neck or something? Heh.
But anyway, LCDs emit heat, since they have circuit boards in them and those little wires do have a finite resistance. The heat though comes from the wires themselves, not from other stuff like an electron gun tube, so it's like your computer's motherboard when you have it on for a long time. Since most people mount their monitors vertically, convection usually does a good enough job of drawing away the heat so that it's not a concern. You should leave a couple of inches of space on all sides though so that it doesn't just recycle the same (hot) air over and over, but gets fresh air.
Just feel your laptop after a couple of hours of use. You'll find that the back of the screen is still cool to the touch, even though the laptop itself is warm. This is because for laptops, the screen part is only the LCD panel, with all the other stuff tucked away inside the keyboard half. For monitors though, they're all in the same place, so the monitor will get warm.