If you've had to track down RSVP stragglers, I think you'll enjoy this...
"Miss Manners... considers it an unfortunate concession that hosts even need to state that they would like a response (in sensible times, guests could figure that out for themselves), and finds it distasteful that they feel they must provide the response themselves, in the form of prestamped cards or a voice-mail drop. Especially since such techniques don't work. People who are not polite enough to answer invitations are still not going to answer, even if you lie down in front of them and beg them to kick you once for yes and twice for no."
--Judith Martin, Miss Manners' Guide to Domestic Tranquility, Chapter 7: Entertaining: The Social Contract, p.218
