Ya you are right. As per what i have seen is visiters preffer to concentrate on the centre of any page which is the mainplace for the matter or the content. After that may be any where else like links and other stuff.
And ofcourle there ould be very les people would be svrolling right down and seeing the footer adds
May be the footer can get you only impresions. I have kept image only add at the footer which may have chances of working.
Rest all depend on the visitors where they see and hat they want to click