Going unconscious DDI does not mention that a creature drops any held items, yet it seems logical to do so.
Is this mentioned elsewhere in the rules that I am not aware of? Has WoTC ruled on this question before?
Perhaps the default answer should be: "Yes, falling unconscious causes you to drop items whatever you are holding, but if an item remains within reach, you can pick it up again when you regain consciousness as a free action."
This allows for speedy recovery to combat without removing the game play opportunity for the item to become out-of-reach (rolled away, stolen, or borrowed by a party member.) When considering these possibilities, it seems unlikely that you should normally hold something while unconscious. Seriously, what if I want to take the scroll-of-save-the-party from you - would I have to make a strength check or something?
You are right it is not mentioned. his sounds more like a situational adjudication by the DM.
It may depend on a number of things. The creature may be able to maintain a loose grip on the weapon/item held by the type of creature, say undead or far Realm, or if the terrain the fall on isn't too difficult. On the other side the terrain might be difficult or unstable like a hill and they tumble/slide a short distance or a swinging bridge, then they are likely to drop/lose a hold of everything they have. It may be that even if they do drop the item, when they regain consciousness they could easily retrieve it as they stand up as part of the move action. Tough call.
I would say that as a group you might want to discuss this one and set a house rule.