I built an underground mob spawner in 1.8 SMP that I can turn the lights on and off in. I used pistons to move glowstone over glass holes in the ceiling - whenever the lights are off the holes are covered.
But...there's light coming in from somewhere! The rest of the room is solid cobble or smooth stone, so I can't figure out why there would be so much light! How can I seal the room in darkness?