If you think you might make another trip to WDW within a year or so of this trip, you might want to wait as long as possible to purchase your DDE membership. Your membership will expire 12 months from date of purchase (not date of first use) so if you order it in Aug 2005 it will expire at the end of Aug 2006 even though you don't start using it until Nov 2005.
If you don't mind using a DDE confirmation letter rather than the actual plastic card for your November trip, you could purchase the DDE about 2 weeks before your trip. They will send you a hardcopy confirmation letter in the mail that you would use as proof of membership (and if for some reason you do not receive the letter in time, you could pick up what you need at Epcot Guest Relations). You didn't say when in Nov your trip is, but say you're going at Thanksgiving, you could order your DDE membership Nov 1 and then your membership wouldn't expire until Nov 30, 2006.
At the moment, they are adding additional time to everyone's membership to compensate for the amount of time it takes for the cards to arrive so you will probably get a 13-month or maybe even a 14-month lifetime on your membership depending on the backlog at the time you place your order.
