According to a recent study of the Association of Research Libraries authored by Karla Hahn, Director of the ARL Office of Scholarly Communications, at least 44% of 80 ARL libraries surveyed are involved in publishing, and another 21% are planning to get involved. That's 65% of the libraries surveyed (in numeric terms, 52 libraries).
This is, of course, an understatement of library involvement in publishing, as one of the criteria for ARL membership is a library's size. This means that the smaller Simon Fraser University Library, one of the partners in the Public Knowledge Project and home to close to a hundred journals, is not included in this survey.
With this support from libraries, the Dramatic Growth of Open Access is sure to continue!
For details and links, see Peter Suber's post on Open Access News.
This involvement of libraries in publishing is a key to Transitioning to Open Access