Hi,
I did see the "same address for shipping" checkbox on your registration form on your checkout.
However, I couldn't see the third step of the checkout as I didn't get the activation email (at least not yet) with the activation link.
So I can't really verify your situation.
Note that I think that you have the "same address" checkbox but not were you expect it. This chekbox will appear at the bottom of the new address edition form, so that if you create a new billing address you can have it as a shipping address too, and vice-versa.
But on the address selection interface, you won't see a "same address for shipping" checkbox instead of the shipping address selection area.