Sony announces in Japan a Playstation Network Pre-Paid Card.

The Sony Playstation 'Ticket' will be available beginning of June at over 52,000 post office ATM machine throughout Japan.
Consumers can choose between a 1,000 Yen, 3,000 Yen, 5,000 Yen or 10,000 Yen card.
The cards contain a 12 digit code that can be used on the Playstation Network to pay for games and other offers in the Playstation Store. With the Playstation Network pre-paid card Sony gives users a choice to pay without a credit-card on the Sony PS3 network.

It remains to be seen if Sony is able to offer similar pre-paid cards in the United States or other regions outside of Japan.
Via this Sony press-release (Japanese).