I am grateful to Alan Cann (http://microbiologybytes.wordpress.com/, among other places, and a member of staff at Leicester) for pointing me to a piece in Nature about this Public Library of Science initiative. The piece is at http://www.nature.com/nature/journal/v445/n7123/full/445009a.html.
PLoS One publishes reports from all areas of science. Before publication, reports are reviewed but only with an eye for their methodology. Anything published is made available for open peer review, that is, anyone can make comments, and the paper can be rated. Papers are viewed as the start of the discussion, as the Nature piece quotes a PLoS person saying, and not as the end. Papers are published on payment of a fee, on the same model as used with other PLoS journals, and journals from other publishers, so that they are then open access.
The Nature piece does articulate the view that this is PLoS' way to make money (it has not been making much, thus far). It also argues that new journals often struggle to attract papers until the journal has an impact factor, and seems to suggest that this new journal may not be able to get an IF, as it "accepts everything". We shall see.