HDGuru offers a nice tip on how to work around deceptive lighting conditions when shopping for an HDTV:
Cup your hands forming a tunnel with your thumbs and index fingers making the front opening. Place the pinky side of your cupped hands against the TV screen and place your eye against the front opening. You will need to find or black area of the picture, if you’re lucky, black bars will be present at the top and bottom of the screen on a letterboxed demo material. This will give you an idea of just how light the blacks are on different displays.