wiring around the fireplace
Let me first say...Do you know how high the mantel is going to be above your fireplace? If you mount the plasma above the fireplace to high then your neck is going to hurt while watching t.v. What I would do is sit if your recliner or whatever you are watching t.v. in and look up for about 15 minutes and see if your neck is going to hurt. It looks really good to have a plasma above the fireplace but will it work in the long run.
If you find that it will be low enough to watch. Then you have to decide what signal you are going to bring to your t.v. Are you going from receiver to T.V. via Component, DVI, or HDMI?
Once you decide what signal you are using then I would run some pvc piping along backside of the wall in the back of the alcove (for future uses, it's a pain to fish wires later in an alcove) Make sure your foundation makes it accessible for you to run the pvc. I would run like 2" pvc. Whatever you do make sure you think this out NOW, because you don't want to have to run cables after the sheetrock is hung. Also, don't forget to run some 12/2 or 14/2 romex for your ac power.
Good luck
I hope this helps