Well a number of sites have been doing this a lot lately, the position of the top bar or <div id = "header_outer"> in
wikihow.com is *fixed* so even after reading the last line of article one does not need to scroll back up. The top bar provides certain functionality easily like searching, help section, log in etc.
I personally am trying to figure out whether this is possible in Wikipedia or not. While reading long articles, it is very tedious to scroll up for a search box or even for accessing the index !!
Would love to hear more on it. Thanks.