We launched our virtual economy this summer and are just integrating premium (the ability to pay for virtual economy coins). We're live in 11 markets with premium and gradually adding more. People use their phone and something called WAP-billing to pay for the coins. $6.99 buys you 200,000 coins for example.
Historically monetizing users on mobile have been an order of magnitude easier than on the web. Ringtones is a billion dollar business for example. SMS is a $80 billion dollar business. What web services/categories can generate that kind of revenue? And SMS is a pretty poor protocol. The key is the built-in mobile micropayment system called PSMS or WAP-billing.
One of the companies we look at for inspiration is Mobagetown, a mobile web based community in Japan. It's a $250MM company in terms of revenues at just 10MM users.
Our main challenge is to make people realize that Alexa doesn't track mobile web ;)
Gustaf, Heysan