Using a canvas screen or tapestry to hide a TV is another old school method for concealing a wall mounted flat screen. The cheap version of this solution features a spring mechanism (like a rollup blind or an old-style projector screen). You can pull the screen down, let go, and watch it whrrrrrzip up to reveal the TV underneath. Or, you can choose a pricier motorized model that rolls the canvas screen into the housing mounted on top of the TV using a remote control.
If the TV has a very thin profile and is mounted close to the wall, this kind of cover-up can actually look OK from the front. But the TV will still be readily visible from the side. If the TV is inset into the wall to let the canvas screen hang flush with the wall, the cost of the whole setup rises substantially. If you are springing for a built-in TV anyway, why not consider a TV lift that drops down from the ceiling rather than putting your TV in the wall?