A few weeks ago, I went to a talk hosted by the IxDA Seattle chapter called "Brave NUI World," hosted by the Artefact Group. At the event, some of the Microsoft Surface team showed off their technology in action and had a panel discussion about how their product's Natural User Interface (NUI) was developed within Microsoft, as well as the user experience and usability learnings they'd gleaned from the 5-year development process.
One of the things that really struck me from this entire panel was how much energy they'd invested into paring away anything that did not look and behave like a real physical object, as opposed to trying to integrate interface elements such as a scroll bar or a text input screen into the device. Everything in their NUI required physics or real-world behavior, which creates some interesting opportunities for interaction, but is also a major limiting factor for accessing any large quantity of information.
Want to see what considerations go into designing a NUI? Artefact has done a good job of videotaping the panel, which you can watch at your leisure here or view them at the original post at 720 px wide:
Another company I saw a presentation from almost two years ago was the Netherlands-based design firm Lust. They crafted a NUI for their design firm retrospective "Generation Random", which is also quite instructive to watch in action. Click in the (not very usable) nav on their site the first text link, to see a video of their first exhibit in action.