You order the DDE by calling their office which is open Mon-Fri 9am-5pm. If you are too close to your trip dates for the card to arrive in the mail, they will leave a temporary card for you at the front of Epcot. It will be there by 5pm on the day you order the card. So if you wait until you are at WDW to purchase the AP and then call to order the card, best case is that you arrived at WDW early in the day on a weekday and can call the DDE office right away. In this case you can pick up your DDE card at the front of Epcot at 5pm that day. Worst case, you arrive late on a Friday, purchase your AP after 5pm and can't call the DDE office until 9am the following Monday morning, losing the use of the card for the entire weekend.
For most people it works out better to order the DDE card about 4 weeks before your trip so there is time to get it in the mail to avoid a trip to Epcot that first day to pick it up. If you want to spread out the cost of the APs, then purchase just one in late September and have the voucher mailed to you so you can use it to purchase the DDE. Since you plan to go back in Oct 2009, you could call the DDE office as soon as your voucher arrives and get a DDE card with an Oct 31, 2009 expiration date. If your Oct trip might extend a bit into Nov 2009, then wait until the 2nd week of Oct to call the DDE office and verify that you will get a Nov 30, 2009 expiration date. Ordering it by the 2nd week of Oct '08 should allow enough time to get the DDE card in the mail for your mid-Nov '08 trip.
ETA: This assumes that what we now know about DDE (how to order, delivery timeframe, expiration dates, etc.) is still true a year from now!