I remember a few years ago I saw a demo of the "Unreal" 3d engine, mostly used for games. It would allow you to virtually walk through a 3d environment. Since then, I have seen some good and bad attempts to recreate a similar engine in Flash. Well...
Florian Krush and Kai Hofmann have developed one of the best 3d environment engines (made in Flash) I have seen.
Check it out: www.mx3d.com.