It's a bright sunny day where I am right now, and I can't get the meter to budge just by covering the top LCD. Whatever little bit of light is reaching the meter that way has no measurable effect on metering with the lens cap off.
Those of you with underexposure problems can easily rule out the LCD as the source of your problems. Just test the camera with the LCD covered up for a series of test shots. If it's still underexposing, then the problem lies elsewhere.
Have you confirmed your camera does it in the ideal conditions mentioned (ISO 800, AV mode, view finder blocked, lens cap on)? Not all bodies are doing it, mine certainly is.