Our wedding is Dec 3rd 2011. We arrange special pricing with our TA that is guaranteed until our departure and the RSVP deadline was Feb 25th. We had 60 people pay their $200 deposit and it didn't seem to be a problem.
There were about 15 people that couldn't commit so early and are ready to pay more come September if they try to book closer to the date, and they are fine with that.
If there wasnt a deadline and we just allowed people to book their own trip, I think there would have been a lag in RSVPs.