I had your exact issue. Had an e-mail from Apple saying my card was declined and the option to change payment method. Put in a different card (had to ask to borrow a parent's and then pay them back, how embarassing) and then everything was fine. It said my new payment method was accepted, and my order is now sitting at "Processing" with the same 9/21 date.
Even more embarassing b/c my total for 2 64GB iPhone 5's for me and my wife came to $1498 plus tax, and my card's limit was around $50 short... I'm selling my unlocked 4S's on eBay so once they sell I'll probably only owe somewhere between $300-500, but explaining all of that to a parent at 7am and asking to use their card just for the pre-auth was a little tough. Lucky I have very understanding parents. ;-)
I made a thread about this earlier asking if anyone knew for certain how the queue handles such situations. My guess/hope is that for a certain period of time (24 hrs?) it holds your spot in the queue to give you another chance to enter a working card. Just curious what the time window is or if maybe there is some other procedure I'm not aware of.
If anyone knows from experience how this can impact your delivery date, etc., I am also curious to find out.