Wanda R. Marsolek et al. have published "The Types, Frequencies, and Findability of Disciplinary Grey Literature within Prominent Subject Databases and Academic Institutional Repositories" in the Journal of Librarianship and Scholarly Communication.
Here's an excerpt:
INTRODUCTION In many disciplines grey literature, or works that are more ephemeral in nature and are not typically published through traditional scholarly channels, are heavily used alongside traditional materials and sources. We were interested in the type and frequency of grey literature in subject databases and in North American institutional repositories (IRs) as well as what disciplines use grey literature. METHODS Over 100 subject databases utilized by academic researchers and the IRs of over 100 academic institutions were studied. Document type, search capabilities, and level of curation were noted. RESULTS Grey literature was present in the majority (68%) of the literature databases and almost all IRs (95%) contained grey literature. DISCUSSION Grey literature was present in the subject databases across all broad disciplines including arts and humanities. In these resources the most common types of grey literature were conference papers, technical reports, and theses and dissertations. The findability of the grey literature in IRs varied widely as did evidence of active collection development. CONCLUSION Recommendations include the development of consistent metadata standards for grey literature to enhance searching within individual resources as well as supporting future interoperability. An increased level of collection development of grey literature in institutional repositories would facilitate preservation and increase the findability and reach of grey literature.