indyank - 3:15 am on May 19, 2012 (gmt 0)
I agree with crobb305 on seeing ranking differences between plural/singular over a year ago. To be precise it was ever since the site was pandalized. Prior to that, pages were ranking for both singular/plural versions of a keyword. It might be that this happens only to affected sites i.e. sites falling below Google's "quality threshold" loose their authority which earlier helped them rank earlier for singular/plural keywords.