Actually Disney has informed us (The Timeshare Store, Inc.®) --"If the contract is submitted for review on or before March 20, 2011 it will not be subject to the changes."
So technically someone could buy on March 19th and if they returned the documents on March 20th along with the seller then it would still have all the benefits as if bought from Disney as long as The Timeshare Store, Inc.® submitted the contact to Disney on March 20th.
Most likely though you will need to buy by March 15th, 2011 to make sure documents can get back by the March 20th deadline.
We do have some pending contracts that can't close until some time after 3/20/11 and those buyers are protected as well as their contract will be submitted to Disney before the 3/20/11 deadline. If you see a contract that you want to purchase and it says can't close until some date after 3/20/11 be safe knowing you can purchase these as well and still get the same benefits.
Jason