If the /replaceitem command is performed with a sign the item isn't displayed. For example: The sign is overwritten by this command:

/blockdata ~ ~1 ~ {s:"s",x:116,Text4:"\"\"",y:104,Text3:"

{\"color\":\"dark_gray\",\"text\":\"*Click*\"}

",z:151,Text2:"{\"color\":\"dark_red\",\"clickEvent\":

{\"action\":\"run_command\",\"value\":\"/execute @p ~ ~ ~ /replaceitem entity @p slot.armor.head minecraft:sapling 1 0\"}

,\"text\":\"Oaksapling\"}",id:"Sign",Text1:"\"\""}

For me it's not displayed that I got a sapling on my head but my teammate can see it.
(Have a look at the pictures for this)