bw100 - 9:58 pm on Aug 18, 2011 (gmt 0)

but if you just keep on scrolling down, their sponsored ads might not remain as much visible to users

Within reasonable conjecture, they might implement a floating right-column ad; ads anchored to the top / bottom of the browser window could replace the top of page ads when you begin scrolling down. Look for dynamic ad copy, as well, as scrolling continues.
Google will not leave any dollars "on the table".