I just tried out the 3D web browser from SpaceTime. It is kind of neat and the 3D rendering and animated fly through are fun.

When you search in Google it will open the first 10 search results pages and line them up in 3D. You then can fly through the pages. This is a good experience, but the delay at the beginning to preload the 10 pages and render them is a turn-off.
Overall SpaceTime is in a gimmick state right now and will not be more efficient than 2D browsing.

The SpaceTime 3D browser was downloaded a 100,000 times in its first week of release in early June. Clearly people are into 3D experiences, but it is still a long way until we will use the Web efficiently in 3D.
NTT introduced a similar 3D web browser called SpaceBrowser already back in 2004.

You can download and try SpaceTime for free on the SpaceTime site. Via Heise (German).

The MIT Technology Review published a lengthy article about the future of 3D and the web dubbed Second Earth. The article looks at different trends and the possibility to merge Google maps with Second Life type virtual Worlds creating a copy of the real world.