The current item is indicated by a class on the body and to stop users clicking the link on the current page a span is placed over the top of the link to stop it being clickable.
Unfortunately IE6 needs a specified width and height on the anchor and on the span to make this work as it gets the position absolute wrong otherwise.
Most other browsers wouldn't need the width and height on the anchor and would accept height:100% and width:100% on the absolute span instead.
IE also needs a background on the span otheriwse it allows the link to be clicked through. You can use a 1px transparent gif or in fact an image that doesn't exist if you don't mind error messages in your server log.