We sent our invitations early and requested that people reply by Sept 22 (our wedding is nov 9). That gave us a lot of time to figure out who was coming and who was not...except for those who have STILL not replied. If you and most of your guests are from out of town, you can use your room block as the guide. Just call the 800# periodically and ask them to e-mail you a list of the names of those who have booked their room.
We also set up a wedding website, not a Disney site, that we e-mailed our guests (and included the link on the save the date...Disney will add your link to their save the date if you ask them to). On the website we had a place for people to "rsvp" not officially, but just to give us an idea. Of course, not everyone utilized the website, but it still gave us a pretty good idea.
We invited 160 and 80 are coming. My general thought - if people are avoiding you, they are probably not planning to come and they want to avoid telling you this!!! Hope this helps!!