We purchased our DDE for our March trip about 3/8. When we got there on March 2/22 the temporary card was waiting for us at Guest Relations (the one outside the park, not the one inside). A few weeks later our permanent card arrived and it is good through April 2007, so we end up getting 11 months (and three trips) out of it.
Please do remember to tip based on the full amount before discount, not the lower amount (in fairness to the wait staff)
It worked well and the discount applies to alcoholic drinks as well.