I started having the same problem yesterday, and here is what I posted to Apple Support:
Hi Apple Support,
I'm hoping you can help me. I have been been getting my credit card declined message when updating my billing method in itunes since yesterday, 8 Sept when trying to buy an app for my iphones and ipads. It was working fine until then, and there were no problems. I am a US Military member stationed overseas in the United Kingdom, with a US iTunes/Apple account with US based credit card(s)which has worked fine up until yesterday.
I did a net search, and my problem seems to be related/described in the following link:
http://www.aeromental.net/2010/08/31/itunes-store-bug-your-credit-card-was-decli ned-please-enter-valid-credit-card-information-2/
To check this, I tried logging into US itunes from my UK based macbook, and setting my billing information and got the credit card denied message. I then remotely logged into my mom's laptop in Tampa Florida (within minutes of the first attempt at home in the United Kingdom), and was able to successfully update my credit card information. I logged out of the US based system, and tried to make a purchase with my newly updated account from macbook at my home in the UK, and was presented with the "update billing information" dialogs again, and then subsequently failed that credit card update. I then repeated the process, and was able to again successfully update my account billing info from the US machine and made a purchase, but the process repeated itself when moving back to the UK machine. I have also checked with my bank, and they only see the "successful" purchase and no declines at all. This is with trying 2 separate credit cards and a debit card. This would appear that there is IP domain checking or geographic checking going on and thus iTunes (not my bank) is denying the credit card update and subsequent purchases. Can you please put me on an "exclude list" to fix this or have this fixed so I can resume being a loyal Apple customer (imac, macbook, 2xiphone4, ipad, 2xipodtouch3G, nano, shuffle, 2xATV, Powermac Dual G5tower, MobileMe.). When I set the account to "no credit card", I can "purchase" free content and updates with no problems. I have also tried this on multiple machines with itunes10 installed.
All these devices are somewhat limited if I can not purchase content through the itunes store.
This is extremely inconvenient, as I was trying to purchase the tom-tom USA application for a trip I leave for on Sunday in Wash DC. I would really like this to be remedied before then. This seems to be itunes issues, as my bank(s) (both of them) have not processed any transaction requests that were denied.
Your speedy reply/assistance is greatly appreciated.