DracoSuave
First Post
@DS: I missed your copy/paste of the errata and when I quoted the compendium you didn't say anything about it being wrong/out of date. Blah. You've got the right RAW, but I'm still lead to believe that RAI was as I described, but for that they'd have to errata the power to take into account the sneak changes.
Even then, it still checks when the attack hits.
The old reading still requires that you be attacking a creature with combat advantage.
'Once per round, when you have combat advantage against an enemy and are using a weapon from the light blade, the crossbow, or the sling weapon group, an attack you make against that enemy deals extra damage if the attack hits.'
If you make your attack roll, then knock the creature prone, then deal damage, you have not attacked that creature when you have combat advantage. You attacked it before the combat advantage occured, and therefore, you cannot with any honesty say that you've satisfied the requirement for this power.