^ Also, later in the film, they accelerate gradually up to warp speed. Sulu counts as high as warp point nine at one point. It's not until they get past that point that we see the warp effect and the ship disappear, thus moving faster than the speed of light.
Now, that could mean one of two things. Either the ship is accelerating at impulse power up to Warp 0.9 before entering warp, or Sulu was not actually reporting on the speed of the ship but on the warp power being generated. He could have been saying, essentially, "we've reached 90% of the power necessary to go to Warp 1" and when they got to that level of power, the ship enters warp.
The intent of the writers/director seems to have been that the ship was gradually accelerating to warp. But there seem to be no other examples of a ship accelerating in this manner, I'd have to go with the second explanation.