Law Professor David R. Cleveland has two new articles out on the status of unpublished federal court opinions: Overturning the Last Stone: The Final Step in Returning Precedential Status to All Opinions, 10 J.App.Prac. & Process ___ (2009)[SSRN] and Draining the Morass: Ending the Jurisprudentially Unsound Unpublication System, 92 Marq. L. Rev. ___ (2009)[SSRN]. Special thanks to the Law Librarian's Blog for its posting on this topic, which includes an interview with Professor Cleveland here.
For the status of unpublished state court opinions, one useful source is: Melissa M. Serfass and Jessie Wallace Cranford, FEDERAL AND STATE COURT RULES GOVERNING PUBLICATION AND CITATION OF OPINIONS: AN UPDATE, 6 J. App. Prac. & Process 349 (2004). The source is a little dated, but relatively easy to update on your own.