Mana Knight of ps3forums stumbled upon PSN Cards at a local Meijer's Store.
The PS3 had no problems recognizing that he had a $20 card by entering the code on the card but it would not go into his PSN wallet.
He contacted Sony to ask about the Wallet issue and was told that the PSN Cards were sold a day early to him but they are officially available on June 13, 2008, Friday.
He was told to clear the cache in order for the $20 PSN Card to be available for his PSN Wallet. We believe it was done through the browser, but this is unconfirmed.
After all the cache was cleared the PSN Card worked as intended.
So for those without credit cards or want to give someone a nice gift, look out for the PSN Cards at your local retailer.
Thanks Mana Knight!
[Source: Sony PS3 Modding - Homebrew, Upgrades, Mods, and Hacks]