OA repositories and their affect on the subscription decisions made by librarians is the focus of the recently-released report by the Association of Learned and Professional Society Publishers (ALPSP), "ALPSP Survey of Librarians on Factors in Journal Cancellation". I haven't read the entire report yet (it's not OA) though I did complete the survey. It seems that there is concern on the part of ALPSP about what the burgeoning amount of content in OA repositories of various stripes will do to library journal subscriptions. The results of the survey may allay these worries, at least in part; for instance, respondents said that availability of articles in repositories was behind faculty needs, usage, and price as a factor in cancellations and that repositories are not seen by librarians as substitutes for "properly managed journal holdings". The data that does make ALPSP sweat a bit is that 54% of respondents said that availability in "OA archives" is an important factor in determining cancellations and that 81% believe that this importance will grow in the next half-decade.
The summary and conclusions, survey results, and press release can be found at http://www.alpsp.org/publications/pub12.htm.