I disagree with everyone else. I would rule that you absolutely get your Sneak Attack damage with that power on a hit.
Yes, by the strict reading of the RAW that states "Once per round, when you have combat advantage against an enemy and hit that enemy with an attack that uses a crossbow, a light blade, or a sling, the attack deals extra damage" it sounds more like you should have to have combat advantage before you attack in order to deal the Sneak Attack damage. However, I think that strict adherence to the cult of apparent RAW is not necessarily the best way to reliably arrive at the intent of every single little rules case. And that's not the ONLY possible way that you could interpret that rule, anyway.
In this case, the way that power itself is written indicates its intent, to me. For one thing, there would be no point at all in listing the prone effect on a separate line before the damage line. None. Most powers would list it as part of the hit line, or afterward. The fact that it is conspicuously placed before is good evidence that it's intended to affect the damage roll after it.
Also, the flavor text of the power (I don't want to hear about how "fluff isn't rules", the power fluff text can often be useful in deciphering how the power is intended to work in cases like this) reads:
"You deliver a flying kick to the face of your foe, knocking it to the ground. You then plunge your blade into its vitals."
It's obvious that the intent of the power is for you to knock the target prone, and THEN deal damage, taking advantage of that prone state. The power may have been written a bit carelessly given the strictest reading of the RAW on Sneak Attack, but it seems clear to me that it was intended to work that way.
As the RAW could possibly be a bit more loosely interpreted to allow for it, and the writing of the power (both in its specific and rare formatting order, and its fluff text description) strongly implies that it is meant to work in such a way that you can take advantage of the prone condition with that very damage roll (which comes after the target is prone), I'd rule with no hesitation that Sneak Attack could be applied in this case.
Some people are more fanatical worshippers of the most restrictive possible reading of any bit of RAW that they can find. I am not, and I believe that most good DMs are not, and that the game designers themselves are not.
I think that the power is written exactly how it is because it is intended that you have no combat advantage for the attack roll, but you do have it after you've hit, but still affecting the damage roll. Denying Sneak Attack here seems to me a poor call, contrary to the intended design of the power, for no better reason than a sort of clinging to one possible reading of a more general rule.