Depends on how much heat there is above the fireplace - if there is a mantlepiece it may deflect the heat into the room and away from the wall. How about putting a candle on the mantlepiece and seeing if it melts - if it does it's probably going to be to hot to put a TV up there.
best idea is to send an email to Sharp or suchlike and ask what they recommend, i did this with a plasma tv manufacturer cos i wanted to place it above the firepace and they said that it would be ok as long as the heat did not exceed 90 degrees at the tv, also its a good idea to have a wider mantel piece to deflect the heat.
plus you have to take into account the viewing angle.
If you will have to look up at the TV, it mihgt not be comfortable. + Some panels look crap at angles other than head-on.