Sony is finally releasing pre-paid "Playstation Network Tickets" which can be used to purchase PSN items. It will likely work in much the same way as Wii pre-paid cards work right now. According to Sony they will be available at over 52000 locations throughout Japan in four varieties. 1,000 Yen, 3,000 Yen, 5,000 Yen, 10,000 Yen. For reference 120 Yen is about $1 US dollar.
No word on a US release but they're set to hit Japan in early June.
Japan is so Rad, I wish I could buy PSN Tickets at my local Circle K.