I threw items onto slabs and no shadow was produced. I then did it on full blocks and there was a shadow - this was tested in a 1.5.2 Bukkit server. I then did the same in a single player world and the problem persists.
Images:
http://imgur.com/a/SXW2l
Top 2 - Bukkit server
Bottom 2 - Singleplayer
Duplicate of MC-21024, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being resolved as duplicate.