Sony have started sending out $20 PlayStation Network cards as a holiday gift, so they should hopefully be available in the US in the near future.
Sony released their new Playstation Network Cards released in Japan yesterday. They work just like the X-Box 360 and Wii cards to make purchasing content that little bit easier.
To quote: Don't have a credit card? Want to purchase things on the PLAYSTATION Store? No worries. Now you have the PLAYSTATION Network card. The kind folks at SCEA sent us an early Christmas gift -- a card that's redeemable for PS3 and PSP purchases on the PLAYSTATION Store. No further details have been officially released, but rest assured that these cards will be readily available in retail stores in the future.