The APA published a new Style Guide to Electronic References in 2007, which updates the relevant section of the 5th edition of the Publication Manual.
On page 2 it states: “For undated or otherwise changeable content retrieved from the open Web, as well as in-preparation, in-press, or preprint journal articles, include the retrieval date. No retrieval date is necessary for content that is not likely to be changed or updated, such as a journal article or book.”