Minecraft 14w18b, Minecraft 1.8
"/setblock ~ ~ ~ minecraft:sign" produces the error message:
"There is no such item with name minecraft:sign"
However, minecraft:sign is an item, and works with commands such as /give.
setblock requires a block, not an item, and so the correct names to use for setblock are "minecraft:wall_sign" and "minecraft:standing_sign", which are the blocks produced by the sign item when it is placed in the world.
As such, it would be less confusing if the error message instead said:
"There is no such block with name minecraft:sign"