Indeed. From the university behind arXiv, this is less of a surprise. Having said that, however, it is obviously a big step and should be applauded by the scientific community at large.
Only "an Ivy" or similar has the buying power and (more importantly) social capital for such a move. (E.g., consider how the elite social network lights up if a particular journal of note insists that therefore they won't sell to Cornell.)
At the same time, the top-50 (or -whatever) institutions have the least incentive to rock the boat: they're usually rolling in "buying power" :) and their faculty are relatively likely to be among the winners of the game as it's currently set up (negative-sum game though it is). I think it's safe to say that if the top 50 were sufficiently motivated to make closed-access journals go away they would already be gone by now.
