bolen: Hey, if it really is the end of the world, might as well spend the final days doing something intellectually stimulating. Plus, in this case, really - it will hit, or it won't and barring interference the fact of the matter has already been decided. Classical mechanics is deterministic. It's just that nobody has bothered to tell us which it is.
Katerek: A couple of technical notes -
Comets lose mass while circling the sun, but asteroids don't (unless they come close enough to the sun to actually melt stone). That sucker can circle for millions of years, and be the same size.
While some smaller asteroids do "detonate" in the air, to a thing that large, moving that fast, the earth's atmosphere pretty much doesn't exist. The mass of the thing won't be heated enough to cause it to explode before it hits, be it stony or nickel/iron. Sorry, but no "high altitude burst".
Similarly, things that size are difficult to "skip" off the atmosphere. It's too blasted big. If it's possible at all, the range of incident angles required is very small.
The ozone hole has little to no effect on asteroid impact. It isn't like the ozone layer is thick enough to absorb a mountain or anything.