I THINK they'll be able to turn 360, but they'll only turn when someone's within a narrow front arc. Would make sense.
As an Engineer, I wouldn't want that. If my turrets can only track targets in front of them, I want to trust that "in front" is where I told it to look. I would be able to accept 360° turn radius if, like TF2 sentries, the turret reset to its original facing after it loses a target.
Personally, I think 90°-120° firing arc, similar to the turrets in ET:QW, would make the game more interesting for everyone. I would have to think harder about how to place my turrets, and the enemy team wouldn't be completely shut down by a turret they way they are in TF2.