Sorry for the eleven-o'-clock news-y intro, but this is critical info for anyone who cares about their gear. Want to know if fireplace placement is safe for your gear? Tune in right here, right now on Unplggd.
Yes, it's safe...
You can check with your manufacturer, but most TVs can safely withstand up to 125°F. Since the TV generates it's own heat you want to keep the ambient temperature around it several degrees below that.
Yet counterintuitively, lighting up the fireplace doesn't makes a ton of difference in the heat around your television. If you can make it out, below is a thermal photograph from a Thermal Imaging specialist on the This Old House boards. When he shot his own over-the-fireplace-mounted TV setup, it showed that the only heat around the television was its own radiant heat.
The rising heat from the fireplace goes elsewhere—up an exhaust and dissipated by the mantle—so your TV doesn't see any ill effects from lighting a fire below.
...but, yes, it does void your warranty.
Sadly, this choice isn't up to science. If Samsung or Vizio decide that mounting your new television above the fireplace is harmful to it's operation and against "basic use" principles, they're within their rights to pull your factory warranty—and most do. You'll want to check in and ask each of your warranty guarantors—the manufacturer, the retailer and even your credit card company, if applicable—to see if fireplace moutning is a no-go for coverage.
[via This Old House Forums]