Harvard Online has an in depth article about marketing to the players (or avatars) of virtual worlds.
Second Life is just one of a growing number of three-dimensional virtual worlds, accessible via the Internet, in which users, through an avatar, are able to play games or simply interact socially with thousands of people simultaneously. By some estimates, more than 10 million people spend $10 to $15 a month to subscribe to online role-playing environments, with the number of subscribers doubling every year. Millions more enter free sites, some of them sponsored by companies as brand-building initiatives. Many users spend upward of 40 hours a week in these worlds. And as the technology improves over the next decade, virtual worlds may well eclipse film, TV, and non–role-playing computer games as a form of entertainment. That’s because, instead of watching someone else’s story unfold in front of them on a screen, users in these worlds create and live out their own stories.