I'm thinking of hanging my 108 pound plasma from two steel
cables (so I can motorize the height). I'm confident I can do it, but before I start I'm curious if anybody has attempted this. My only unresolved issue is the attachment to the rear of the set (sammy pn50a650).
Any ideas?

No experience just a cup of coffee. Most wall mounts come in two parts, a bracket that bolts to the Plasma and the wall mount.

So you buy the wall mount and throw the wall mount away and you have a sturdy bracket for attaching your cables. You may need to drill some holes in the bracket.

The only problem is the center of gravity is going to be all wrong and the panel will tilt forward. However you may be lucky and the amount of tilt is just what you want.

If not, you will need to make a more sophisticated bracket that goes up the back and extends over the top edge of the plasma so that when you hang it the center of gravity pulls it straight.

I suppose I would catch two eye bolts at the bottom and pass the steel cables through two open eye bolts at the top that I could adjust for the best angle. One cable gets an adjustable turn buckle behind set to level the unit.
The whole idea is to be able to have the unit in FRONT of my fireplace for a comfortable eye level when fireplace is not in use.

As long as the cable hanging down from the ceiling sits behind the center of gravity of the Plasma it will tip forward a bit. You could experiment with a couple layers of plywood the size of the plasma, a block of wood the thickness of the depth of the bracket, and some eye bolts. See if you can come up with a configuration where it doesn't tip.