Here is my best guess. For me, it just started slightly after May 7th. Prior to that - no problem. Same credit card, same address, nothing changed on my end.
Something changed on iTunes end to affect us.
I think that they have enlisted a mailing address verification firm to ensure there is less fraud - of some internal reason. When they pulled the trigger, some of us - for whatever reason - were tagged as unverifiable.
It's not the credit card. If they were using just that, the card companies are OK with our addresses. So they have added another layer - and that layer is the mailing address.
IF - they would explain what has changed, and what they are doing - it would help us to make data sync. WE cannot repair the problem if we don't know what they are doing.
Anyway - that is my take on the matter, and I currently have no idea how to be able to update my apps, much less buy anything...