I am having the same problem in Hong Kong. My iTunes account was created when I lived in the States (with an US credit card and billing address). I have been working in Hong Kong since 2006 and spend most of the year here.
Prior to the problem, I was able to purchase both music and app on both of my computers (work/home) and my iPhone 3GS.
After upgrading my home computer to iTunes 10, I started getting the credit card problem and e-mailed iTunes Store support right away on September 5th. In the meantime, I was able to buy music on my work computer, which still ran iTunes 9.
On September 14th I e-mailed support again because I still have not heard back, and Brian got back to me right away saying he had not heard back from the engineering team either.
Then last night he e-mailed me saying the problem's been fixed - so I tried buying music, and it works on both my 3GS and my home computer. However the problem started happening again when I tried buying an app.
After reading the recent replies of this thread, I think that the act of buying an app reset my account and threw me back into the loop, and now I am unable to purchase anything from the store.