I was wondering the same thing about the $20 credit. I did a live chat with someone from Amazon, and they looked up my order. They said, "I've checked the order and see that $20 promotional credit will be applied to this order." Apparently, as long as you pre-ordered prior to the change from $20 to $10, you will receive $20. NHL 12, release-date delivery, and $20 promo = good deal.
You will get the $20 credit. The promotion changed for the orders placed now. I see it was $20 when you placed. So you'll get the $20 promotion credit. I see the order placed using the two day shipping so you'll get the order on Sep 16, 2011
^^^Sweet deal on the no cost upgrade to release date delivery for you. I chatted with a custom rep and was told I too will still get the $20 credit. I had already upgraded my shipping from standard to release date for the extra dollar.
Check your e-mail. You would have received an something saying:
Your recent order xxx-xxxxxxx-xxxxxxx entitles you to a promotional credit which we have added to your account. This credit can be applied to your next qualifying purchase.
It doesn't show up anywhere, that I know of, until you try to buy something. If you really want to check, put something in your cart and checkout, but don't actually purchase it. You should have an option to apply the promo credit.