I do not quite agree with the statement that "agenda" has been widely accepted as a singular noun although it is a latin plural. Indeed, agenda is a latin plural but an English singular. The latin singular "agendum" means a (!) task to be done. The latin plural thus refers to tasks (!) to be done. The English word agenda, however, with its meaning of a set of problems that need to be addressed/dealt with = a list of issues definitely has a singular meaning since it denotes THE LIST rather than the issues themselves (as agenda in latin). This is very unlike the word data, which in fact means gathered facts (!), thus is clearly a plural.
Yours, hathaway

There is a difference between data and agenda. Data in English has the same meaning as in Latin: facts (plural) gathered. It, therefore, cannot be used with a singular verb. Agenda, on the other hand, has in English (A LIST OF tasks to be performed/items to be discussed = singular) a different meaning from Latin where it means tasks (= plural) to be performed. Thus, using agenda with a singular verb is not a matter of being “widely accepted.” It is simply correct while using a singular verb with data, also “widely accepted,” is not correct.

agenda (pl. agendas): a list of things to be done/discussed (etymological plural of Latin agendum)
data [plural; singular form datum]: facts that have been gathered (from Latin datum, past participle of dare = given]
(Source: American Heritage Dictionary and Longman Advanced American Dictionary)
"Certain nouns in –a are regularly treated as singular, though the ending represents an original plural: agenda, insignia. The use of other nouns in –a as singulars is controversial, e.g. media, criteria, phenomena, strata etc. In scientific discourse, data is commonly used as a non-count noun" (= followed by a singular verb). (Source: Greenbaum, Oxford English Grammar)