When we ordered our invites we also ordered rsvp cards (Disney themed of course ). We ordered ours online, the site below actually has the ones that we ordered. Our rsvp asked for how many would be attending and their meal preference (beef/chicken/fish/veggie) so that we could provide Disney with an accurate number of each meal requested.
A few weeks after Disney sent out the STD letters we sent out of invites and rsvp cards. However we sent ours out VERY early as our guests were suprised (they were under the impression that it would be a local event not a destination event) that it was at Disney so we needed some clarification and such. Our guests then had a few months to consider rsvping. However those who we knew would come rsvp'd early, and those who were slackers we knew probably wouldn't (and didn't) attend we had to prod for a response near the deadline. HTH.