As soon as you have a Kindle assigned to your account you can start ordering books. So if you order NOW, within a few minutes you'll likely have a Kindle showing on your account. . .even though you don't physically have it. This is true even if you order a DX that you won't get for maybe a couple of weeks. Once Amazon has the Kindle attached to your account you'll be able to buy Kindle editions.
If you try to buy a Kindle edition and you don't have a Kindle, Amazon will kindly offer to sell you one.
