It looks greeeeeaaat. And from my point of view it was posted at the right time - I'm also planning something similar. I currently have a brick fireplace which is being extended out by 20cm in a similar way to your structure.
I'm also extending the recess below the plasma area which will house the amp and dvd etc. Luckily for me my wife doesn't want a fireplace feature.
I'm also looking at buying the Panasonic PWD8 42" and I'm going down to Kent home cinema on Tuesday for a demo. How deep is the recess for the Plasma? Have you considered the size of the wall mount?
I'm not a handy man so I've ask a local carpenter to provide a quote for the work. Hopefully I will get this on Monday and start the work within 3 weeks.
Look forward to seeing the end results. I will also provide some before - during and after photos for the fourm members.