John Langford recently blogged about researchers preference to cite recent research. He calls this tendency "the forgetting" of prior work. John suggests a number of reasons recent work may be remembered (including "Dead men don't reject your papers for not citing them").
John also points out the obvious: forgetting is a bad thing. It may lead people to overestimate the value of a paper, and it reduces the efficiency of contribution. He wonders whether our line of research may be able to help: "Wouldn’t it be great if all the content at a conference was organized in a wikipedia-like easy-for-outsiders-to-understand style?"