I inquired about this with Disney Visa and you can pick up a rewards card on site. First, you'll need to call Chase once you're at WDW. Then, you'll be able to pick up your rewards card on site. If memory serves me correctly, you can pick the card up at MK and in DTD. I would call Disney Chase before you leave just to double check. I would hate for you to show up and due to changes or something not to be able to get it after anticipating it. Oh yeah, you'll need to have your Disney Chase card in hand with ID to pick up the rewards card.
I have time to order mine, but I plan to wait until I'm there for 2 reasons. First, i want to accumulate as many points as I can. Second, I want to make sure I need the rewards before I order the card. From what I have read, the rewards card expires in one year of issue. I have some Disney Dollars and Disney GC I plan to use first.