Whenever I use it. I can be in the middle of a game and it dims slightly like the brightness got turned town. I'm also using a 26" HD LCD TV BTW.
I honestly can't think of anything it would be apart from your backlight dying. If you felt like it you could try another monitor as there is the smallest chance it could be GPU, and you could also look through the menus on the TV to see if there is anything there, although I doubt it. Unless the monitor actually has a room brightness sensor and is adjusting it accordingly (very unlikely for a TV), it is most likely the backlight.