It doesn't get as hot as you think above a fireplace. We flush mount plasmas all the time above fireplaces and have no problems. I would recommend a ventilation system with fans though.

Ernie, I know the Sony 400 disc will fit in a 24" cavity. I believe it is 21 or 22 inches from the back of the "Hump" to the front of the knob. We tell our cabinet guys to leave us at least 24" to make room for wiring. I wouldn't go less than 24" though.