Does the 80/20 rule still apply?

As I've been studying the usage of our patron-driven acquisitions (PDA) program, my mind has turned to the Pareto Principle of resource usage - the so-called "80/20 Rule".  This principle, initially developed to describe the distribution of land and wealth in Italy (according to Wikipedia, of course), has been used to describe distributions of all kinds of resources and effects.  It was first applied to usage of library resources by Richard Trueswell in his 1969 article in Wilson Library Bulletin (I'm still trying to track down the original article via Document Delivery).  This is a rule that was presented to us in library school, and one that I've always wanted to investigate, but never (until now) had the chance.

In an effort to apply more critical thinking to my work (see Paul Wyckoff's advice to politicians), I'd like to consider the evidence that supports (or does it?) the principle.  I've started a literature review to answer these questions:

• Has it ever been supported by evidence from more than one source?
• To which collections did it apply the most?  The least?
• Has the application of this principle changed over time?  
• Does it still apply today?  If so, to which kinds of collections?

If it can be supported by evidence, then I believe libraries and librarians are going to be in big trouble.  In this day and age of accountability and return-on-investment, we are not likely to be able to support the budgets that provide resources for which only 20% account for 80% of the usage.  

Some bigger questions I have include:

• How does this principle affect the dissemination of knowledge?  If a large percentage of the resources are not used at all, how can the information and ideas be spread? 
• Has the Internet changed this principle?  Does the medium matter less than the message?
• What effect does the increase of scholars have on the availability of published material?  As the pool of resources has increased over time, has rate of usage declined?
• Can the "publish or perish" model of academic tenure continue?  
• What will be the impact on scholarly communication of changes to collection development that result in fewer items being purchased due to greater emphasis on usage and less on preserving the scholarly record?