I was thinking about the mail also or worse... products on shelves in our stores that are not tamper proof.
I was thinking about the food angle first ...but would be problematic in that it would be hard to get everyone to eat the same type of item at the same time and harder still to contaminate vast numbers of food items throughout the country simultaneously. (Perhaps on a traditional holiday certain types of items might be targeted, I suppose (ie: Thanksgiving).
A mailing though could be done simultaneouly to lots of people and could involve an attractive offer or sample product that might even invite someone to smell it (fragrant powder, perfume etc).
I hate it when my mind works like this! (/paranoia)