Here’s an article from the entertainment section of CNN.com about ABC lifting its ban on showing video of the planes striking the World Trade Center. The article says:
TV network executives promise to be judicious in how much they use video images that are already burned into the collective memory. But it’s clear those images will return. Nearly a month ahead of the anniversary, CNN began airing an advertisement that shows one of the planes heading directly toward the tower. The scene cuts away before impact, however.