Energized's explanation is consistent with my experience. I have a factory moon roof on a Toyota and it has drains that run along the perimeter of the moon roof. Water has never leaked in to the car. However, these drain lines can get clogged which would cause leaks that appear in the foot wells of the front seat (the water basically starts dripping down the outside of the lines along the inner walls of the car and collect in the wells). If there is water dripping from the ceiling though, I think that's a bad moon roof. You could check on those drain lines though.
