In response to Tom's request for one university that will guarantee that they collect all their research output, here are two:
Queensland Institute of Technology, Australia, since 2004. University mandate since 2004. http://eprints.qut.edu.au/ Now in its 5th year! University of Queensland, Australia, since beginning of 2008. Now achieving annual government research reporting through their IR. This implies 100% coverage of course. http://espace.library.uq.edu.au/ A considerable number of Australian universities, including my own, are independently going down this latter track. Not that in neither case is it guaranteed that 100% of deposits are open access, but it is guaranteed that the deposits are made. In some cases expiry of an embargo will deal with access to the deposited full texts. Equally, there is no guarantee that the figure is actually 100%. It may be 99% with a few recalcitrant or lazy non-performers, but that is generally acceptable. The really active researchers are usually the compliers. Arthur Sale University of Tasmania