From a comment on PLoS blog http://www.plos.org/cms/node/26#comment-22:
As Liz mentions above, part of the PLoS ONE mission is to facilitate debate
and discussion of published research, through facilities like the ability to
post comments on PLoS ONE articles.
One difficulty with this is that the proliferation of places to post comments
on published research can make finding all the comments on an article quite
difficult. For example, comments can be posted attached directly to the
article on the PLoS site, or at Postgenomic, in one's reference library at
CiteULike or Connotea, at HubMed, one's personal blog, and probably other
places that I've missed.
This can be solved only through some service providing the ability to add trackback links (could be a specialized annotation or even a wiki page)