http://leaky.bukkit.org/issues/1052
The bukkit devs are aware of the problem. Until they fix it, I'd advise against storing your most valuable stuff in chests. Making diamonds and iron into blocks and placing them is probably the best way to go for now.
SMP Sign and Chest Wiping
SMP Sign and Chest Wiping
The first law of thermodynamics states that you don't discuss thermodynamics.