You must have a mantle above the fireplace to direct heat away from the screen.
If you are just building right now, then I would ask that you may want to consider how many times a day/week/year that fireplace will be on and how often the TV will be on and which truly should be afforded the higher priority.
Both, is the obvious answer, but if you can't give up either, then why not go ahead and spend a bit more to get a fireplace which is more condusive to TV placement above it?
That is, they have non-traditional low, wide fireplaces which are available. You get the nice fire, but instead of being stuck with a TV that is 4+ feet off the ground, you are able to maintain it at a lower viewing height and still keep your fireplace.
I'm NOT a fireplace expert, but I'm about to do some research on them to be able to make recommendations to homeowners and construction companies, but I did dig up this model, which is in line with what I'm talking about...
http://www.fireplaces.com/en/Products/Cosmo-SLR-Gas-Fireplace.aspx
That site has a fair number of models which offer some more modern possibilities.