The user interface for PCs hasn't changed much in 20 years. It's still windows, folders, icons, palettes, mice. Many UI improvements have been made over the years, especially by Apple, but not major innovations. Lately, Microsoft is showing off it Avalon graphic engine with a 3D user interface for the Vista desktop. Sun has been forever showing off Looking Glass, now an open source project, but it's not shipping with any product. It's mostly eye candy. The Mac OS doesn't currently use a lot of 3D effects (like windows floating in 3D space), but it applies many cool and useful animation effects, such as Expose, with its Quartz graphics engine. I'll predict that Apple's next rev of its OS will include more visual effects for modeling the desktop, subtlely blending 2D and 3D.
Project Looking Glass
Eventually the kinds of deep 3D interfaces used in games will merge with the staid desktop, but the major owners of user interface franchises, and most users, prefer evolution to revolution in the design of the user experience. There isn't much incentive to force users to adopt radically new interface conventions. For a look into a whole new way of interacting on a computer, check out Project Croquet. It's innovative open source operating system and computing environment developed from the ground up by Alan Kay and a team of researchers.
At the opposite end are the Web platforms that avoid unnecessary graphics that can slow down and clutter the online experience, but eventually the lack of bandwidth constraints, and as everything converges on the Web, will force them to compete with more rich user environments. But users should have a choice of what kind of interface they prefer. Standard conventions will ensure that you can move easily between 2D and 3D environments. First, there needs to be more competition and innovation around creating the next major breakthrough in user interface. If we are depending on the big players, it will take decades to happen. I am interesting in hearing about innovative UI projects, so send them my way....