Sony announced that last month was its biggest ever, as far as revenue generation from PSN goes- but if they want to keep it that way, they really need to fix this problem. Apparently, in the wake of the new Flash Sale that Sony is holding on PSN, people are complaining that the PSN Store simply will not accept their credit card information- in many cases cards that were working on PSN just fine until recently.
The issue has cropped up on multiple game related forums throughout the internet, including NeoGAF. In the past, the issue could be resolved by simply editing the address on file until it was identical to the billing address for your card, but that seems to be ineffective this time.
For now, the best thing to do is probably just getting PSN Prepaid cards. And really, given Sony’s patchy track record with security, I would be hesitant about getting my credit card information on their servers in the first place.