From the forum:
In talking about a team, for example, Los Yankees, should a singular or plural verb be used? Los Yankees son or Los Yankees es?The use of plural verbs is standard with teams that have plural names. Thus, for example, a headline in today's El Universal of Cartagena, Colombia, says: Yankees derrotan a Azulejos 9x6 y siguen empatados con los Orioles. (Yankees defeat Blue Jays 9-6 and remain tied with the Orioles.)
In preparing my answer, I did run across a few cases where a singular verb was used; for example, one headline on the Univision site used "Yankees derretó"; perhaps the headline writer was thinking of the Yankees as a singular team, as the article itself used a plural verb. In any case, such a use of the singular is an exception. The use of the plural verb is clearly standard use as well as what grammarians would recommend.
But your question does raise another issue with regard to the verb ser — there are definitely cases where you'd say "los Yankees es." To give two recent examples, Vida Latina San Diego recently wrote that "Los Yankees es el equipo que más gente va a ver" (The Yankees is the team that most people go to see), and the current version of Spanish-language Wikipedia writes: "Los New York Yankees (Yanquis de Nueva York en español) es un equipo de béisbol" (The New York Yankees are a baseball team). That's because in Spanish, as explained in our lesson on ser and agreement with the predicate, it is common in Spanish for the form of ser to agree with what follows, not what precedes as in English. In other words, it isn't Yankees that is acting as the subject of es, it's the singular equipo, even though it may look otherwise to an English speaker.