Two of the shots featured in the trailer that left us stunned were those featuring a moving Iron Man suit clearly not being operated by Tony Stark. But the context and nature of the Extremis virus offer an explanation that could be incredibly inspired.
On a broad level, the Extremis 'virus' heightens brain activity and cognitive speed, making Tony Stark's subconscious far more active than a normal person's. Add the telekinetic connection with his armor, and Tony's subconscious suddenly has a physical form to take, and a means to manifest itself.
The suit is first seen pulling Pepper Potts (Gwyneth Paltrow) off of a sleeping Tony, having possibly assembled itself in his sleep to defend him (is the ensuing fight that leaves the armor scattered at the bedside?). Following the destruction of his Malibu home and Pepper being placed in danger, Tony is shocked to see the armor looming over him, prepared to fight.
If this really is the relationship being hinted at - between Tony and the parts of himself he won't acknowledge - then the possibility for character development and confronting demons is truly exciting.