Through some visual effects magic the teaser attempts to link all 11 Doctors together, with the 11th himself, Matt Smith, doing a nice job of narrating what the current season arc (and arguably the entire show) has been building to. While they sell most regenerations of the Doctor rather heart achingly short (1,3,4 and 10 are prominent) it's a nifty little way to get us amped for the special.
Have a look below.