Yes, the big thing here is not getting people to whip out a credit card. When you are some random kid its much harder for you to impulse buy something that requires your mom to plunk her credit card number into some random system.
What I like is innovation in use.
- Mochi is teamed with a rewards site so that people can do tasks on the web to earn rewards points, which they can then use to buy upgrades or other microtx when they see fit.
- Habbo Hotel and other interactive sites have prepaid cards available inside stores such as Target or Best Buy so that kids can spend their cash on the cards instead of credit.
- Webkinz integrates their online purchases with real-life stuffed animal purchases, so that parents are willing to get you that toy and you also get the online codes.
I think there are ways to do it, but convincing the user that investing a few bucks into your game when there are so many other free games out there is rather difficult. You have to have some giant hook or great game to really make some money beyond banner ads. It's been done, not easy, but doable.