The virtual world ... uses a wide variety of games to teach kids how to manage their money, hone their math and science skills, and even learn how to eat better.
Nedra at Spare Change has recently described her visit to Second Life. There she finds the nonprofit world busily establishing itself. In a post on Immersive Marketing you can find the complete blurring of entertainment and marketing in virtual worlds as practiced at Neopets.
If you are thinking about developing health promotion and social engagement applications in virtual worlds designed for kids and tweens, or conducting research on these issues, perhaps Whyville might make a nice center of gravity.