Hmmmm. I just remembered this topic, and I was thinking, since this object is travelling at ~0.9c, you might be correct that it would vaporize upon hitting the atmophere. But I don't think this would diminish its destructive effect. This just means that the entire energy of the object (which is considerable) would be tranferred into the atmophere instead of the ground. So there wouldn't be much of a crater, and there wouldn't be much in the way of a dust cloud (or at least not as much of one), but you would have a truely gigantic shockwave travelling around the earth.
I have no idea how to calculate the damage that such a shockwave would do. Anyone care to take a shot at it?
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