OK MOMS!!! this RSVP thing is one of my biggest pet peeves. PLEASE RSVP. If you get an invite let us know if you are comming or not. I have had this problem as well with 3 boys, lots of parties and events each year and people are to rude to reply.
what I do is I started useing www.EVITE.com as well as sending a paper invitation. this way those who are too lazy or busy to call can email their reply. It also lets you send updates and reminders...Evite is a free service!
if I still have not heard from people about 4-7 days prior to party I call them on the phone and see if they are comming.
You could also include a prestamped post card in their invitation with reply information they can check and just drop in the mail.
I hope this info helps I know how frustrating it is to try to plan a party and people dont reply.
Good luck, A. J