This really isn't anything new but interesting none the less: The Hidden "F" In Your Web Pages
A team of scientists engaged in military research recently came to the conclusion that when it comes to reading web pages,most readers read in "F" shaped pattern taking into consideration the eye movements of 232 readers,it was found that a reader generally reads in a horizontal movement first.This is followed by their eyes moving downwards a bit and then,ensues reading across in a second horizontal movement covering a shorter area than what it was previously.
A good part of this just natural with English written from left to right. The other part I would imagine as conditioning, with the majority of web sites on the Internet conforming to a few relatively standard layout options viewers just get accustomed to what they are supposed to do.