My favourite theory is the Steven Spielberg Effect. The dinosaurs wanted to make sure they didn't stay around to end up in movies for little kids.
But seriously... I guess the most likely cause of extinction was global cooling or global warming associated with dust clouds from an impact of a comet or a giant meteorite. Though I also like the lastest theory that the atmosphere was much thicker at the time of the dinosaurs. For some reason, the density of the air decreased and the dinosaurs couldn't cool down in the thin air--and died out. A change in vegetation around the world might not have helped, either.
But will we be wiped out?
Well, there is a tiny chance that we will be wiped out by a comet or a meteor splatting into the earth.
But it's far more likely that we can do the job ourselves. Nothing else will have to do it for us. We've got plenty of possibilities--over-population, pollution, environmental deterioration, ecological collapse, nuclear war, savage mutant viruses, genetic engineering gone wrong, the ozone layer being zapped...and probably more horrors that we haven't even invented yet.
I personally think, though, that we might turn the corner away from destruction and learn to improve things a bit.
I know that sounds naive but I think humans have the intelligence and the desire to do it. Especially if we learn to love one another a bit better at the same time.
I think Buckminster Fuller was right when he said a long time ago that, if humans are going to survive, then it will only be because we make the Earth a wonderful place to live in again.
Utopia or oblivion.