Here's an excerpt from the press release:
Royal Society Publishing is converting all its journals, hosted on HighWire Press, to a continuous publication model. This initiative emphasizes the fact that the online version is the authoritative, most complete and up-to-date record, and ensures peer-reviewed papers can be cited immediately.
The introduction of a continuous publication model is a logical step forward from the current 'publish ahead of print' feature (known as FirstCite) and will provide many benefits for the scientific community: researchers will have full citation details available upon publication; an author's published article will accumulate citations without delay; and journal impact factors won't be skewed by articles whose FirstCite and issue publications span two different years. Continuous publication also means that page numbers will no longer appear within a citation; instead, each article will have its own CrossRef-compliant, unique identifier, found near the top right-hand margin on every page of an article.