what happened with mine was actually not apple's fault, although there customer service is still rubbish and they could really do with better error messages. Because of the nature of the itunes billing system (sending small packets of costs) my bank for some reason thought someone had nicked my card and was using it fraudulently. After i rang the bank up, they took down their 'firewall' which was blocking transactions from my account at that point, and i can now use itunes again. So if all else fails, ring your bank up and ask if they are blocking your card.