sorry for posting in such a old topic but this came up in google search while i was looking for something else and very confused as to why you all think Unity 3D is a virtual world platform like opensim and second life.
If you all go to unity3d.com you will see that it's a game engine, NOT a virtual world platform. Even though it can be used for virtual worlds, the main focus for Unity developers is to make a engine and editor for game developers and make it easy to public those games onto anything from windows, mac, linux to consoles and mobile devices. It's mostly used as a client side engine that stores and generates all the 3d models, npc's and scripts. It does have networking that requires the developer(s) of a game to either create their own server code or "buy" prebuilt Unity supported server code from other developers.
In short Unity 3D is NOT a virtual world thing like OS and SL, its a game engine to help developers make any kind of game.