NIH's public access policy is now mandatory for all NIH-funded investigators. The policy requires submission of a full, electronic version of each published manuscript to PubMed Central, where the full text or PDF is freely available for viewing or download. The BioMed Central blog has a good post about it.
This might be the biggest push towards open access to date, with a government mandate covering the majority of US researchers. Not surprisingly, many publishers are not happy with this development, but I don't think there's much they can do about it, since they will lose a significant number of authors/manuscripts to Open Access journals if they try to deny them, and with that will go a significant amount of their prestige.
Now, what would be even more helpful (and perhaps PubMed Central is doing this) is to provide free access to the full text of each article. Even better, free access to structured text of each article. Natural language processing of biomedical text is a growing field that would benefit hugely if sections, figures, tables, captions, references, etc, were all labeled as such in a computer-readable way. Maybe it's not such a pipe dream, even, to imagine all articles structured this way and indexed with biomedical terms as an automatic pre-publication step.
It will be interesting to follow the drama between Open Access, traditional publishers, and the NIH policy in the next few months!