I totally hear you on this one. We are getting married May 5, 2011 and our RSVP deadline was 4 weeks ago and we still have 27% of our invites are still outstanding.
The most frustrating thing about it is that most of those are to very close family and friends (like my fiancee's siblings and all of my fathers siblings and their children). We had asked people to RSVP to both our DW and our at-home-reception in the same card - which was an ordeal in itself, since we've realized that most people DON'T READ instructions.
We are sending out the information cards for the At Home Reception, so for those people that haven't returned their RSVP (or called or emailed us) we enclosed a small "friendly reminder" card to let them know that the deadline has passed and that they can still call, email or send back the RSVP. Lets hope this works.
Honestly, I just don't get it since I'm pretty sure that most of the late responders are not coming, I'd think that it would be a lot easier to decline by mail rather than having to answer us point blank over the phone.
Oh, one last thing... we were at a friend's gathering the weekend of Feb 25th and asked one of the couples whether they were coming, since we hadn't yet received their reply. They said, "well I figured you knew were couldn't come to the Dominican Wedding". How can people be so dim? My response was, "how would we know that? Of course they had no answer for that - we still haven't gotten their RSVP yet!! ARGHHHHH it is so frustrating!