From what I can work out, it is the contrail of the SRBs. Have a look into the shuttle trajectory, the launch site is visible to start with in the downward facing camera. Then, the view moves, and the sea almost fills the view with the launch site towards the bottom of the frame. In order to insert into the correct orbit, the shuttle "rotates", and does not go straight up - if fact it only goes straight up to clear the tower and any flight paths. The launch is really a parabola, in that an orbit is a parabola its just that is goes around the body you are orbiting. As it rotates, you can see the SRB exhaust gases, and the water vapour of the main engines as the classic launch plume.
I also noticed that the SRB is still "firing" for a quite while after it seperates - watch for the clouding on the lense from the exhaust gases and the flames you see later from the motor end.
