Stupendous. I have a hard time imagining that this isn't the turning point in the OA battle, at least in the U.S. and certainly in the biomedical sciences. The U.K. already has Wellcome and others with an OA mandate. The mandate (assuming it's enforced or researchers are worried about it being enforced) will encourage the infrastructure of OA and cause spillover effects. By infrastructure here I don't necessarily mean institutional repositories, as those are fairly pervasive at this point at major institutions, but rather largely the knowledge of how to self-archive among researchers. If they have to do it for half their papers and discover that it only takes two minutes (and eventually they find distant colleagues contacting them who never would have otherwise), that dramatically lowers the barriers to doing it for non-mandated (e.g. non-NIH-funded) articles.

--Ari