Hélène Prostand Joachim Schöpfel have published "Degrees of Openness: Access Restrictions in Institutional Repositories" in D-Lib Magazine.
Here's an excerpt:
Institutional repositories, green road and backbone of the open access movement, contain a growing number of items that are metadata without full text, metadata with full text only for authorized users, and items that are under embargo or that are restricted to on-campus access. This paper provides a short overview of relevant literature and presents empirical results from a survey of 25 institutional repositories that contain more than 2 million items. The intention is to evaluate their degree of openness with specific attention to different categories of documents (journal articles, books and book chapters, conference communications, electronic theses and dissertations, reports, working papers) and thus to contribute to a better understanding of their features and dynamics. We address the underlying question of whether this lack of openness is temporary due to the transition from traditional scientific communication to open access infrastructures and services, or here to stay, as a basic feature of the new and complex cohabitation of institutional repositories and commercial publishing.