Not sure if there is an adult equivalent but there are sites which allow users to get into your site on credit and they pay it back at a later date and if they don't pay it back their credit score goes down so they can't get into any more sites on credit.
I think I remember seeing something like this on GFY the other week, so somebody might chip in.
Edit: Found it!
http://www.gfy.com/fucking-around-and-business-discussion/992481-shap-question-free-found-money.html
Though I think SMS billing is the way forward, Global-Access and Itelebill are the ones I know of