Minecraft wiki entry for Development_versions, associated text: Removed the item forms of many blocks: bed (26), piston head and extension (34 & 36), redstone wire (55), wheat crops (59), standing and wall sign (63 & 68), wooden and iron door (64 & 71), lit redstone ore (74), unlit redstone torch (75), sugar cane (83), cake (92), unpowered and powered repeater (93 & 94), locked chest (95), pumpkin and melon stem (104 & 105), nether wart (115), brewing stand (117), cauldron (118), lit redstone lamp (124), tripwire (132), flower pot (140), skull (144), and unpowered and powered comparator (149 & 150).
They can no longer be obtained with /give, but can be created with /setblock.
Because the game is still told to drop a lit redstone lamp when mining a lit lamp with Silk Touch, doing so will result in a Null Pointer Exception crash as such an item no longer exists.