The annual pass will not "start" until you've activated it. So, you can purchase now and, use it in October and it'll be valid till the following October.
With the DDE card, that (in theory) becomes active when you order it. However, they do give you a little extra time in the length of the card.
If you plan on going again in October '06, then I would try to order the card at the beginning of October. In theory, the card is supposed to expire at the end of the anniversary month that it was purchased. So, if you purchase on October 1, 2005, it'll be valid through October 31, 2006.