It has been reported in many search engine optimization discussion forums that Google is now using word stemming, or treating the plural version of a keyword the same as a singluar version of the keyword. Google seems to be slowly introducing word stemming as the results of our tests show. Generally, the Top3 listings under the plural and singluar keywords are the same but subsequent listings tend to differ.
It is still too early for the SEO community to announce with authority that Google is totally using word stemming. I think they are slowly introducing word-stemming or at least experimenting with its effect. It does not appear to cover the entire algorithm though.