Credit card is the only way as of now. As far as I know they dont have gamecards planned anytime soon. One thing you could do is go to your bank and get one of those visa giftcards and just use it as your game card. I know Bank of America offers them, its pretty much a gift certificate in the form of a Visa/Mastercard credit card, that can be used anywhere those two cards are accepted. You can just keep adding more money to it.
I know of a few people that do this for FFXI, just because you dont have to be 18+ to get/use one as you do with a real credit card.