There are attacks that target an area (bursts, blasts, etc.); if I use an AOE attack and hit multiple enemies that I have combat advantage against, do I deal sneak attack damage to all of them?
From the Compendium (emphasis mine):
When you make an attack with a light blade, a hand crossbow, a shortbow, or a sling and hit an enemy granting combat advantage to you, that enemy takes extra damage based on your level (see the Sneak Attack table). You can deal this extra damage only once per turn.
Sneak attack can only be applied once per turn, and is applied when you hit the target. Once you hit one of the targets and apply sneak attack to it, you no longer have sneak attack available to apply to other targets that round.