I remember reading about this innovation not so long ago. It will be usefull when applied to certain applications. For example, it is a cool gimmic to use when giving presentations. I can just see all the swanky ad agencies and design houses using it for client presentations.
However, for a desktop environment one big problem I see is that it effectively cuts down the useful viewing area of the browser to be about a quauter (or less) of the available real estate. That's not very usable! In this case it might be more appropriate as a navigation aid in a seperate window.