Eye tracking sounds very elegant when one talks about studying a sites overall usability. However I found that it is very challenging to conduct. Part of the challenge is the technology itself is very fickle. Even on the $50,000 eye tracker we used there were still calibration problems, and bugs in the software. Another part of the challenge comes when creating an experiment design and trying to decide what to study. Often times we would think of a cool idea only to realize it something that could be studied with a traditional usability test (no need for eye tracking equipment really).
Below is a presentation I made using Prezi about eye tracking which goes over some of the basics and analyzes the trends and what may be in store for the future. Use the little arrows to navigate the presentation, full-screen mode is also recommended to get the full feel: