I had the same issue over the weekend and the Sony rep first asked if I'd updated my firmware. I am thinking now they might have meant 3.50 because i was up to date the other day and it didn't matter. Their next option was to completely delete and re-type all my account info on the ps3.
I was even trying to add funds via PC and none of this would even effect it (you'd think). But regardless it didn't work.
Their last option, of course, was to buy a PSN card, which I ended up doing. I hope the 3.50 fixed this for future purchases.
I have had the same problem this month whilst trying to add my new credit card.
Have just spoken to my bank, and they were blocking it.
Each time I tried to set the card up, Playstation were pinging my account for a small amount of money. This isn't uncommon practice where companies are trying to verify accounts.
Unfortunately, this is also a common way that fraudsters try to verify accounts, and they frequently use phone top-ups and gaming accounts to do this. They check for a small amount at first (so as to try going unnoticed), and then ramp up the amounts they try to claim.
In my case, my banks auto fraud prevention software was blocking Playstation. I got them to temporarily lift the block so that I could register my card, and it went on first time this time.