Lauren Bigelow has authored a great piece in iMedia called How your brand can click with a tough audience.
It nicely sums up guidelines for using virtual goods to connect brands with the booming audiences in virtual worlds.
At WeeWorld we've been working with brands for years to deliver compelling and fun campaigns and we've learned a lot in the process. Fundamentally, it boils down to fun. Use your brand to bring on the fun and the audience will respond and promote it to their friends, amplifying brand loyalty and favorability.