Well, I was being half-flippant. There is a basis for arguing that Tolkien does not use Deus ex Machina at all, or at least that he uses it in a credible and therefore justifiable manner. Here's an interesting thread with some further discussion on the issue:
Deus ex Machina
Obviously, Jackson has carte blanche to use the Army of the Dead, as they are in the book. And, as in the book, their use is foreshadowed by Aragorn and co's journey through the Paths of the Dead, so they do not simply appear unannounced (which is, on one view, a requirement of a "Deus ex Machina"). To simplify things, he included them in the Pelennor. I have no problem with that per se. But I have some sympathy with the criticism based on the manner in which they are used. They are simply too destructive, so that they end up winning the battle and saving the day, rather than the men of Rohan and Gondor.