Many educators are beginning to hear about the potential for 3D learning environments. Perhaps the best known emerging platform is SecondLife. For all it's promise and interest, early adopters of the SecondLife platform find many problems and difficulties as a "ready for prime time" platform. Part of the problem relates to the question of openness. Linden Labs, developers of SecondLife, has been struggling with the inevitable challenge of moving towards a more open framework in order to encourage a broad development community, while maintaining their early advantage of a "for-profit" business model.
To familiarize yourself with SecondLife, particularly in education, check out their wiki site. Also, Check out NMC, a leading consortium of educators who are experimenting with virtual worlds and simulation in education.
Finally, Sun Microsytems' DarkStar project shows some real promise towards overcoming the practical problems of "bandwidth" and processing power that currently makes working with virtual world environments klugey in classroom settings. Sun's design creates a peer-to-peer network of user computers that overcomes the processing limitations of a server-client network in much the same way as bittorrent does for large video downloads and streaming. The MPK20 Sun Virtual Workplace demo, while graphically uninteresting as an early version of the technology, nonetheless shows some of the promise of 3D virtual worlds in education.