Does posting the preprint break the journals embargo policy?
No, Nature already allows authors to post preprints on recognised preprint servers; the same policy applies here. Nature will not be publicising any preprints to the media or otherwise commenting on them until the final version is published. We cannot exclude members of the media from signing up for the e-alerts, but we will state in our sign-up instructions that the e-alerts are for the scientific community only, and that the preprints are not intended for media comment.