Heat dissipation would be my main concern. I have a friend who built a new fireplace to incorporate his plasma tv but didn't allow for easy access to the TV when they needed to change cables etc. And he ended up modifying his hard work to enable him to get access.
Wall mounts are available that allow you to move the TV out from the wall help with cable access.
Premier Mounts are very good have used them in a couple of projects. They now seem to be available from a variety of UK stores.
This mount from Premier may be of interest to you.