I think that Mendeley are a fascinating example of the Open Access problem. OA is about moving knowledge from researchers' private environments (their laptops, hard disks, CDs and filing cabinets) into the public space (repositories, websites, search engines). Mendeley's software spans both those environments - bibliography management for the desktop feeding researcher profiles and CVs on the Web.
As Victor Henning pointed, Mendeley are part of Cambridge's JISC DURA project, which aims to take advantage of Mendeley's position bridging the desktop/Web to try and encourage more public repository deposits. This is a very interesting proposition: maybe a users of a such a service will be more inclined to make their work Open Access? Perhaps the simple act of buying into the "Mendeley proposition" will cause them to be be more favourable to Open Access than they would otherwise have been?
From the outside it's difficult to understand the extent of Mendeley's penetration into a University. What is visible is the public profiles that Mendeley users have created. Although the Mendeley API doesn't allow searching for users, I have been able to identify 53 public profiles from the University of Cambridge through Google (and a lot of manual verification!) Incredibly, only TWO of those 53 researchers have any existing deposits in Cambridge's institutional repository.
This is potentially great news: Mendeley's software has gained takeup from users who aren't repository users. They aren't preaching to the converted, they are getting new users to work in the open, to start to make the transition from the desktop to the Web.
But the OA battle hasn't been won yet. Of those 53 profiles, 21 contain no publication information, and of the 32 list their publications, only 9 have made any of their publications open access through the Mendeley service (a total of 40 PDFs).
The social bibliographic approach that Mendeley are promoting is a promising way forward. It's offering people something that they haven't seen from the repository, but it's not a principally Open Access offering, and it's no silver bullet for providing open access. Commentators who have suggested that repositories are old-fashioned, and that everything can be solved by Web 2 solutions, are being over-optimistic. Repositories are hard work because changing researchers' working practices is hard work and I guess there's no single magic solution that's going to make that effort disappear!