The trailer uses a bit of clever CGI trickery spliced together with old Doctor Who episodes to create what was first thought to be a second bite at the 50th Anniversary event. It’s also using a bit of “look at the 3D I’m doing and not the shots I’m faking” work if you will. You can turn off the 3D by using the gear in the corner of the video window, by the way.
Here’s another fan-made trailer purportedly from the BBC:
Equally as awesome.
You can check out a great piece on how the fake trailer was made over at WhatCulture!.