I am getting married in PV at Dreams Resort and Spa on January 24, 2012, not at Caletas, but I'm sure we all have similar experiences.
We sent out a STD magnet to all of our guests (around 200 people) in August 2010, we did not ask for an rsvp on this. We had our website info on the STD and had a lot of details on price estimation, resort etc (price stayed the same-we changed locations and resorts though lol).
We sent out Invitations in February 2011 (11 months out) and asked that all deposits through the group be made by March 25th to our TA (for those who booked - this would be they're rsvp), we set an official RSVP date for October 31st so that people could find last minute travel deals if they wanted. To save money on postage (and because maybe half of people actually send their rsvp's back) we decided to have our rsvp set up on our website.
As of right now we have around 60 guests that will be joining us in January (around 40 through travel agent, others have time shares they will be using and some on their own). So I guess we had just over a quarter of our invited guests who will make it.
My TA said that its generally less than half of all invited guests who end up going.
We personally did not send out "B list" invitations (second round) as we found this to be a bit rude (from my experience with friends who did this- they always find out) so we decided to just invite everyone in the first go, this is just my opinion though.
Hope this helps. If you have any questions feel free to pm me.