Mine is 130cm (about 51") off the floor, wallmounted to the chimney breast. I have all my AV kit in an alcove to the side and I put up a fake picture rail to hide the cables.
As for height (the real question ;-), it depends on your viewing distance. I think most people recommend the centre of the screen being level with your head in your normal viewing position. Mine is higher than that, and if I was 6 feet away, I would get neck ache, but my sofa is 12 foot away and it is like being in the cinema (I'm looking slightly upwards). I also have another sofa at right angles to the screen, and I find that laying on my back to view from here also works a treat.