Kohl's set up shop in fashion-focused world Stardoll, while Sears turned to the quirkier Zwinky--but both companies saw a tremendous demand for their virtual wares, according to eMarketer senior analyst Debra Aho Williamson. "Stardoll members purchased 1.8 million items from the Kohl's store within its first 16 days," Williamson said. The retailer honed in on Stardoll's female tween users with Abbey Dawn, the clothing line designed by pop star Avril Lavigne.

According to Jeremy Monroe, director of business development, North America for Sulake, it's important to craft branded merchandise with both the look and feel of the world and the preferences of the user base in mind. Finland-based Sulake is the parent company of Habbo, a teen-focused world that boasts more than 106 million registered users in 32 countries.

"We've worked with over 200 brands globally, including Nokia, Burger King and Paramount," Monroe said. "And it's important for us to make sure that their merchandise or campaign is aligned with a particular interest or activity in the user community. We do a lot of research with our members, so we know which brands, TV shows and celebrities they're interested in. That list helps prioritize who we're engaging with."

Paramount Pictures established a grand presence in Habbo Hotel (as Habbo's world is called) for its film, "The Spiderwick Chronicles," complete with quests, contests and a ton of virtual merchandise, including themed furniture. "It was completely ingrained with the story and characters," Monroe said.

Meanwhile, in August, Sears launched a two-story virtual boutique in Zwinktopia, which sold more than 850,000 items in its first 16 days. The retailer also hosted a fashion show, allowing the "Zwinkies" (as the members are called) to get more involved with the brand and its merchandise. "A cool new fashion event hosted by Sears is going on NOW at Lexi Hall!" wrote Miss Cosmopolitan, one of Zwinky's dedicated bloggers. "As an added twist, you'll be able to let users know how you really feel as they strut the catwalk."

Allowing for and expecting members to give their feedback is another must for brands that want to incorporate virtual merchandise into their interactive or promotional media mix.

"Our users constantly ask for branded items," said Lauren Bigelow, general manager and SVP marketing of WeeWorld, the virtual property that evolved from an IM-based avatar application to a full-fledged world in early 2007. Since then, advertisers like Skittles, Cover Girl and AT&T have introduced their own merchandise into the world so that members can feed, clothe and style their WeeMees to their liking.

"They say things like 'Bring back the Cover Girl makeup' and 'Make a Halo 3 poster for my room'," Bigelow said. But they also let you know if a movie bombed or if they think a game or contest is stupid, and you have to be ready to hear that."