Last week Bing announced that the ad matching logic used in Microsoft Advertising adCenter has been updated.
Advertisers bidding only on the singular or only on the plural form of a keyword, adcenter will serve ads for both singular and plural forms of the keyword. adCenter will automatially deliver the plural version of campaign keywords even if advertisers don't already have the plural version included in their campaign.
For example, if your account has the term :car" but does not contain the term "cars," adCenter will now apply and serve the “car” keyword bid and ad copy for “cars”. The same matching behavior will apply if you bid only on a plural form of the keyword, adCenter will still serve ads for both singular and plural forms of the keyword.
Bing was careful to note that only a subset of keywords where the meaning of the keyword is preserved in the singular and plural form will experience this matching update.