At the Seven Stones blog, Thomas Lemberger, an editor of Molecular Systems Biology_, discusses work by James Evans of the University of Chicago in Illinois ("_Science 321, 395–399; 2008":http://www.sciencemag.org/cgi/content/abstract/321/5887/395) that shows how electronic publication has shifted citation patterns. Scientists now cite fewer papers overall, and tend to largely limit their citations to recently published work.
See here for a discussion of this Science article on FriendFeed.
Martin Fenner has written about this topic in this blog post.
(Also posted in the From the Blogosphere column)