According to Paul Hemp in Avatar-Based Marketing on-line worlds are the next frontier for marketers, both in the virtual worlds themselves and the avatars (identities) that users adopt in those worlds.

Some quotes:

many of Second Life’s 100,000 or so residents are highly involved with this place. And that
makes it potentially a dream marketing venue. Instead of targeting passive eyeballs, marketers here have the opportunity to interact with engaged minds. Commerce is already an integral part of Second Life. Residents spend—in Linden dollars, the local currency, available at in-world ATMs—the equivalent of $5 million a month on resident-to-resident transactions for in-world products and services. Certainly, introducing real-world brands, in some form or another, is a logical next step…

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.

Listen to the podscast interview with the author