I did a very simplistic test of this. I did not implement a software solution. Rather, I simply captured a video of myself gazing, for a few seconds at a time, at each corner of the screen of an iPad 2. I cropped the video to my eyes only, and examined it manually. Using the pupil and "screen glint", I could very easily determine which corner I had been gazing at during any given frame of the video.
"screen glint" refers to the shiny reflection of a bright computer screen on the cornea and/or pupil of the eye. It shows up quite distinctly as a bright rectangle.
In the case of this experiment, the screen glint was approximately a quarter the size of my pupil. (To put it another way, the screen glint width was half my pupil diameter.) I did not check how many pixels wide the screen glint, or my pupil, was. Such a measure would go towards determining just how accurately this might be used. (i.e. by determine where the "centre" of the pupil is, within the screen glint.)
I expect that determining the quadrant of gaze would be less accurate with gaze points nearer to the centre of the screen. Nevertheless, it seems that the in-built front camera of the iPad 2 could be of some use in tracking what screen quadrant a user was looking at while using an application.
Thus, it would likely not work well for determining what features of a portrait that a user was examining, but perhaps what view-panes within an application a user was paying attention to.