If you purchase directly from Disney, you can make ressies immediately. Not sure what happens in resale, but I think you have to be sure the deed is in your name first.
Here's our experience, from this year:
- We closed on a resale contract on October 30th
- We were in DVC system 7 days later
- Our points were posted in our account the next day
- We made our first reservation on November 27th
Since we booked our 1st reservation for an October 27, 2007 check-out date, we had to wait until November 27th (home resort, 11 month window rule). While we could have booked day-by-day a few days earlier, we had no problems making our ressie.