Well as there is a damage roll you add all the modifiers to the roll, but see the below Sneak Attack description. It's a bit silly as you obviously hit them, but according to the rules you didn't "hit" them. There's a lot of effects that trigger on a hit (magic item powers are a big one IIRC), so this distinction is necessary. Because there was not attack roll required, there is no hit made.
SNEAK ATTACK
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.