Copypasta from the updates:
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. You can deal this
extra damage only once per turn.
Looking at the power, it goes
Attack: blah vs blah
Effect: blah
Hit: Stuff.
So, executing these in order...
Attack comes first, so you roll the attack roll. Whether or not you hit is determined here.
The Effect knocks them prone.
Then, if you hit, the Hit line deals damage.
Sneak attack checks when you attack with a specific weapon and hit an enemy granting combat damage. You hit or miss when you make an attack roll, not when you resolve the -results- of the hit.
Knocking them prone after the attack roll (and therefore the hit is made) will not effect the damage dealt. And again, you resolve things in order. Roll attack, knock them prone, deal the damage. This power might be written the way it does, so that you can force them prone without it affecting sneak attack damage, but before you deal damage so that if it has a reaction to damage that causes it to shift, it cannot use that reaction.
The Hit line does not determine when you hit, it determines what you do IF you hit.