This use of so is pretty old. I am so tired of hearing people whine about it. The first glimmers of opposition came about when folks started using it in the negative, as in, "that is so not interesting," where, e.g., "that is so intersting" wouldn't raise the smallest cilli. I would suggest that what *really causes the objection to something like "that is so last millennium" would be the use of "last millennium" as an adjectival phrase.