Not sure if it was a coincident or not, but many people who has an iTunes Store US account had encountered the "Your credit card was declined. Please enter valid credit card information" issue shortly before the launch of iTunes 10. I personally encountered this problem several hours after I began using iTunes 10. It began when I tried to upgrade an app within iTunes. I am presented with the following message in a dialog asking me to enter my iTunes Store credentials. After doing so I am presented with my Billing Information and another message to ask me to verify my billing information. So far this is not unusual, since iTunes sometimes does this to prevent fraud. It normally requires the user to enter the Security Code for the associated credit card; if that is what you use as payment. After clicking on the Continue button to commit the information, I am presented with another message. You are now stuck in an endless loop and the only way to get out is to click on the Cancel button. Doing so will prevent you from performing the purchase you want in iTunes Store.
This issue appears to be affecting many iTunes Store US customers who has associated either a credit card or PayPal to their account as billing information.
I had called Apple iTunes support and their only suggestion is to send an email using the button at the iTunes Support page in the section: Get Customer Service -> iTunes Store Account and Billing -> Managing your Account. Click on the "Email Us" button and fill in the accompanied form.
I hope this issue is resolved soon. Since this is now preventing me from purchasing anything from the iTunes Store US or upgrade any of my apps.
[Updated: September 3, 2010, 12:21] Thanks to a Twitter friend, @natesaint, he suggested that changing the payment option to "None" will at least allow the user with this issue to update the apps. Unfortunately, I had already reported my issue to Apple and received an email reply from them telling me that they're working on my issue. I do not want to change anything in my payment details to effect their investigations.
BTW: So far this issue are mostly reported by users outside of the continent of USA. I had also checked that using proxy service does not resolve the issue.
[Updated: September 3, 2010, 13:34] The proxy service (VPN) that I use is Hotspot. If anyone tried other VPN or found a solution please leave a comment below.
One of my reader found that waiting 24 hours resolved his credit card declined issue. Unfortunately, I will have to wait another 24 hours to see if that helps.
[Updated: September 3, 2010, 14:00] Another Twitter friend, @elwoodchu said that HideIPVPN does not help either. This is very likely an Apple issue rather than an IP filtering or true credit card issue.
[Updated: September 5, 2010, 12:00] All of a sudden I can now update my apps without falling into the above mentioned endless loop of credit card declined messages.
I received an acknowledgement from Apple on Friday that they are investigating the issue, but have yet to hear any conclusion as to what is happening. I hope this recent sign shows that they are working on it.