Disney usually starts you out with a full set of current Use Year points, unless you are purchasing new construction but let's not get too complicated here. With a September UY as of yesterday you are in your 2010 UY so a contract purchased any time between now and Aug 31, 2011 would begin with Sep 2010 UY points.
Dues are pro-rated the first year starting on the later of your purchase date, the start of your initial Use Year and the occupancy date of the Unit you purchase. If you purchased today at a resort that is already open your dues would begin today (or possibly tomorrow) so you would pay 120 days worth of dues for 2010. You would pay full dues for 2011.
If you waited until July 1, 2011 for example you would still start with a full set of Sep 2010 UY points but you would pay no dues for 2010 and pay dues for 2011 starting from July 1, 2011, approximately 6 months worth of dues. So by waiting until the middle of next year, you would save 10 months worth of dues but receive exactly the same number of points over the lifetime of the contract.
Just be careful about over-optimizing this strategy. If you wait until just before your next UY starts to purchase the add-on, they might run out of points for your UY. It has happened to a few people who wanted to add on during the last couple of weeks of their UY.