Recombu does a nice piece of analysis on why it is possible.  And even why it would be a very Apple thing to do.

Apple has historically adapted niche input methods and popularised them for a mainstream audience. It did it with the computer mouse in 1984, and then again with the touchscreen in 2007. Both technologies had languished in esoteric devices, or in the case of the mouse, in the labs of Xerox Parc. To imagine today’s PC without a mouse is unthinkable. The inclusion of eye-tracking tech in the company’s forthcoming tablet would be Job’s magnum opus. What better flourish to a career that began with the popularisation of windows, icons, mouse and pointer than to usurp them all?

And there is a bit of substance in the a patent application in place and reported purchase of eye tracking units from Swedish technology company Tobii. 
Overall to me I'd be surprised if the iSlate launches with real eye tracking.  I'm still waiting for decent voice recognition before I buy into an electroencephalographic headset to finish my powerpoint presentation.

  Interesting Eye Tracking Links:
 

http://www.simpleusability.com/services/usability/eye-tracking/demo – interesting video of eye tracking controlling a mouse
http://www.uie.com/brainsparks/2006/06/13/eyetracking-worth-the-expense/ – some of the learning of eye tracking research / heat mapping on website usage
http://www.youtube.com/watch?v=Jd3-eiid-Uw&feature=player_embedded – classic video of headtracking's impact on the Wii experience