The Internet Solution
Virtual environments can be built in any "Metaverse" in the internet. For the visitors of the room in reality there must not be a question like "is it Linux or Mac" and so on. Chris Mennel for example uses the eldest metaverse, "Second Life", on a Windows platform. For open-source-fans lots of private servers with SL-like-interfaces are under construction. But the pleasure of interacting with 50.000 to 70.000 avatars online at the same time will be reserved for some years to the original "Second Life" world.