It may be a window, or it could be that they are showing the angle from the bridge which shows part of the saucer.
If this is the case I always thought they should do that, same as showing the engines when showing a rear view (which they started doing in Voyager and Nemesis, unless they showed that view some other time which I havent noticed)
Im hoping this is what they are doing, especially seeming it looked like the Kelvin had no such window at the front meaning how did they see outside
Edit: Red_Ranger beat me to it =]
In the "far" pic of the Kelvin, there appears to be a front dark slit flanked by small round shapes....perhaps a "windshield" with "portholes" on either side...