One of many 3D metaphor projects
3D interfaces, 3D desktop metaphors, have been investigated in the past by quite a few of the big players, including Sun (Project Looking Glass), Microsoft (e.g. Task Gallery), and Apple (e.g. Multi-dimensional desktop). When you then include smaller companies, there are a hell of a lot more companies who have tried to come up with a useful 3D interface too. Even my small company, Abstract Worlds Ltd, toyed with the idea with some back-burner projects in the late 90s and early 00s with our PC and Java phone 3D engines, but we were unable to create anything useful at that time, pretty yes, useful no.
With mobile phones of various screen sizes, developing in 3D does have benefits even if you present a mainly 2D interface, because it means you are not hardcoding to particular screen sizes. We found this benefit especially useful with our Java phone game in 2002 Strangemaze 3D since one version would work on various phones various screen sizes.
It also makes sense that if you are limited by small screen sizes, as you are with mobile phones, then if you cannot expand x or y then make use of the virtual z axis. However you can expand x and y, it is called panning or scrolling, and that along with zoom, can make the z axis redundant even though you are giving the user the impression of going backwards or forwards depthwise with zoom. Perhaps the most useful application of this kind of 3D to date is the iPhone's multi-touch drag and zoom, which isn't 3D at all.
But who says you have to use full 3D at all (one of the mistakes of VRML which was full 3D) to give the user a type of 3D experience (the original raycasted 2.5D Wolfenstein or Doom anyone? 360 panoramic images, etc)