We booked ours through AAA and since it goes through WDTC we got 0% interest for 6 months. Since costco takes you through WDTC I would assume it applies as well if you use the disney visa.
I also called back and booked airfare through AAA/WDTC and got 0% interest as well.
Added:
I forgot to mention our very first trip we booked with our disney card, did not get applied. I called memberservices and they were able to tell it was a disney vacation and applied the promotion.