I have just had a look through BNC and COCA. It seems that both singular and plural verb forms are equally common after 'most of + group noun' in both varieties of English, so my feeling (post #2) is clearly only a personal one.
"Most of the class is very smart" = we refer to the group of smart students as a whole.
"Most of the class are very smart" = we think about each smart student (as individuals).
It just depends on what you are trying to express.