Not sure how it is with newer flat panels, but old tube type TV's are much like light-bulbs. Most of the wear on them is from starting up. They will actually last much longer if they remain on constantly.
The TV in my bedroom has been on almost non-stop for the past 12 years, only going off during the occasional power-failure or blown circuit breaker. It still works perfectly. Even if I'm watching a show or movie on my computer, I leave it on and just mute the sound.
The TV's in the living room and my mothers bedroom are only on when being watched. In the same 12 year period, we've replaced the one in the living room 3 times, and her bedroom 4 times. In fact, the one currently in her bedroom is the old one from my bedroom that I've had for 20 years now...