working exemple with starting and ending point here:
http://jsfiddle.net/z7E9u/1/
I copy paste basic code here
var fadeStart=100 // 100px scroll or less will equiv to 1 opacity
,fadeUntil=200 // 200px scroll or more will equiv to 0 opacity
,fading = $('#fading')
;
$(window).bind('scroll', function(){
var offset = $(document).scrollTop()
,opacity=0
;
if( offset<=fadeStart ){
opacity=1;
}else if( offset<=fadeUntil ){
opacity=1-offset/fadeUntil;
}
fading.css('opacity',opacity).html(opacity);
});