I agree, Bran - as a long time committee-man (on all sorts of committees!) I have never come across the use of 'agendum.' I guess you are right Trivia, that it refers to each item on the agenda. And I most certainly have fairly often come across the use of 'agendas.' I suppose it has now been used aften enough to count as being "part of the language."

I'm immortal until proven otherwise