With the cost/performance ratio of plasma I wouldn't want anything between my eyes and the display. I'm going to go out on a limb and assume that this mirror will be framed or otherwise affixed to the wall so as to (attempt to) hide the fact that a plasma is behind the mirrored surface?

If I were entertaining this I would be investigating a guillotine lift to pull the mirror up when the plasma was in use. (In "Utopia" it would pull the glass up only, and leave the frame in place around the plasma's screen.) I saw this used recently in an industry magazine where the projector was hidden by a guillotine'd mirror and the screen came out of the crown moulding on the other side of the room above the mantle. It was an extremely nice install.