I had a friend mount his LCD TV in his newly made above fireplace cavity a few years back. That said he just used the built in speaker (which didn't sound so nice) and externally mounted on the wall speakers in the room.
In Florida we had a fireplace with about 20 something feet of stone going up to the top of the ceiling with some granite base. The house (originally built in the 1950's) and purchased around the 1980's was torn down and we built a new home in its place around 2000. We opted out on the fireplace and built a kind of short 1/4 sized multiple shelf do what with deep cavities. The center was designed for an LCD TV. Above is all with a with a bay water view. I had the contractor put PVC (multiple) tubing between the center and the side shelves. The TV cavity has a small shelf maybe some 6 inches high. Underneath is the center speaker. A bigger LCD would fit in the cavity though with no space for a center speaker. The front two speakers are in the shelved on each adjacent side. The are just regular shelf speakers and sound nice. The subwoofer and rear channels are built into the walls and ceiling. Much LV cabling goes from this area to the center of house wiring closet. (a huge mix; video, cat5e, RG6, audio etc) Its a "great room" with high ceilings.)
In Indiana I helped a friend with his LV wiring. He built a great room in the middle of the home. Fireplace stone goes up two floors in height; its nice looking. That said he wanted an LCD TV and my recommendations were to put it on another wall such that when sitting near the fireplace he would enjoy his fireplace and when sitting in the MM section of his great room he would enjoy his TV. Here the ceilings were too high for in ceiling speakers so we did a half wall height external speaker thing relating to the cabling. Speakers are not installed and I am not sure if external speakers will look ok.
Went to a party a few weeks back. It's a retired person's home (a bit big at around 8 K SF for one person to live in). That said its a builders spec home with Control 4 automation (she wants it out). She has a great room also in the house. Here I saw LCD TV's almost on every wall in the great room; sports bar like. But also like paintings all over the place showing art screen savers. Not sure if I really liked that. Mix and match on the sound.
So I guess you can buy a smaller than 55" TV and build or sit it on a shelf such that you can mount a center speaker above or below the LCD TV?