Nature Publishing Group has posted some very enlightened instructions for authors in their License to Publish. Authors retain copyright, AND NPG has included this language actively encouraging self-archiving:
When a manuscript is accepted for publication in an NPG journal, authors are encouraged to submit the author's version of the accepted paper (the unedited manuscript) to PubMedCentral or other appropriate funding body's archive, for public release six months after publication. In addition, authors are encouraged to archive this version of the manuscript in their institution's repositories and, if they wish, on their personal websites, also six months after the original publication. In all these cases, authors should cite the publication reference and DOI number on any deposited version, and provide a link from it to the URL of the published article on the journal's website (see publications A-Z index).
Kudos to Nature, and thanks to Peter Suber for the alert.