Prepaid cell phone plans extend beyond credit-challenged: "Traditionally, prepaid cell phone plans represented an undesirable market of consumers who couldn't secure a regular post-paid contract because of shaky credit. But as the number of Americans with cell phones continues to rise, more companies are promoting prepaid packages to teenagers, businesses, infrequent "glove-box" cell phone users and first-time buyers, wireless analysts said."

Here in France (and more generally speaking Europe), many teens use prepaid cell phone plans. It's worth noting that SMS usage is strongly related to them. Solutions that let teens pay for online content and services are - to the limit of my knowledge - not widespread yet. Paypal, for instance, can't replace a credit card since it's limited to people 18 and older. See also Cash, cheque or IOU?.

Update: thanks to Robert Loch for pointing to Splash Plastic (FAQ), a pre-pay web card available in the UK.