As Fridikk said: you'd need some pretty decent weight to counteract the torque produced by moving such a heavy object along various planes.
For eg: when it extends out so the screen clears the foot of the bed, that is the most extreme bit of leverage. In fact, with a screen that could weigh as much as 100 lbs, the leverage exerted on the mount would probably be in the neighbourhood of 400 lbs. Heavy enough to lift the bed with you on it.
As for cat's nailing down idea: sounds like she used finishing nails and to carry this amount of weight, you'd need a thousand or so. Remember: this is a live load (meaning the weight moves) as opposed to a dead load, like a water bed.
A way around having to fasten the unit to the floor would be to build a steel frame with rubber feet (to prevent any movement) and then line the extreme end of the frame with lead plates or pellets (kind of like the counter balance weights used in drafting boards).
I've researched small high torque motors for other uses and the ones for this one will run you some big bucks. This setup isn't a cheap proposition but the wow factor is worth it.
A cheaper (although a less WOW) would be to install a thin chest at the foot of the bed and just use a TV lift. Where it would pop out of the cabinet vertically.
