Removed the torches from a Zombie spawner in an SMP server, and the spawner just disappeared. The torches were placed on all four sides of the spawner itself. Did this with two different Zombie spawners and it happened on both.

Edit: According to YouTube user gamemaster676 (see the linked comment under the video: http://www.youtube.com/watch?v=I8blg0dH-PE&lc=G7Zr3W8Kk1Y0235siukOjqPd8o8plU8q4x8Dcj5tNk4), this also happened to him with an obsidian block when he removed a torch placed on that block.