Plasma`s don`t like heat and obviously heat rises from the fire bellow so its not ideal, But there are a few people on here who have them above fires.
So as long as the fireplace shelf sticks out it should shield it enough if its only on for odd occasions.
As MAW says, ive often wondered about stiff necks..... i would have thought you want to mount a plasma as little above eye level as possible else you will a) get a stiff neck but b) make the screen look smaller..... does no one else agree with this logic?
Mine at eye level, I would not risk it on chimney breast as I use mine in winter months.
Also if you have children - I do - I wouldn't recommend a Plasma on the Chimney breast, might not give you stiff neck but ret assured children will get stiff neck as they have to look even higher then us adults.
To each their own, everyone has different needs I suppose, but I for one would not be keen to have it on the chimney for three reasons; too high, too hot, and too ugly, I don't want to make the TV the 'centrepiece' of my sitting room. Of course everyone is different and if it works for you that's fine.
Keeping it at child friendly level (but ideally out of reach) is a good idea though. When my son gets a bit bigger I think I'll wall mount and put some furniture in front, keep those sticky fingers and letter openers away!