Do you believe that AI poses a threat to humankind and to the planet Earth as a whole? Why,why not?

I really don't see how humans can last for all that long if their electronic civilization endures. I doubt it will be any Hollywood-esque "killer robots vs. people" scenario, more of a progressive obsoleting of humans combined with a largely consensual merging of ourselves into AI, with some threshold eventually being hit where these whole biological body things seem more trouble than they're worth.

EDIT: As for Earth, I don't see how or why it would be endangered, aside from the possibility of a self-replicating nanomachine terrorist incident, if that counts as AI to you.