I'm the lead developer on a browser-based MMORPG ( http://www.domainofheroes.com/ ) that uses the same "free-to-play, micro-transaction upgrades" model. We're still in open beta right now, so everything that we would charge for is free, but the upgrades we sell are convenience and customization options like increasing your inventory or customizing your chat appearance. I hate item sales personally, so we don't have that option, good or bad.

The game mentioned in the summary is not the first game in the U.S. to embrace the micro-transaction free-to-play model (not by a long shot), but it is true that this model is becoming increasingly popular. We just got back from the Austin Game Developer's Conference and that was the hot topic on everyone's lips. "Are subscriptions dying? Is F2P the future?"