There's a vision in Microsoft which was recently discussed at MIX '08 by Daniel Makoski (Interaction Design Manager, Microsoft Surface) called Natural User Experience (NUE). The vision of NUE is to, "deliver intuitive, seamless experiences that unfold through natural human input." Part of the NUE is the Natural User Interface (NUI), which is the interface a user would interact with. Microsoft Surface is a prime example of a NUI. Think of NUI as the following: CLI -> GUI -> NUI (and if you want to take it a step further, which I won't delve into at the moment because we won't be seeing or hearing of it for quite a long while) -> OUI
CLI (Command Line Interface) = (Think textual, such as the command prompt or DOS)
GUI (Graphical User Interface) = (The Windows UI)
NUI (Natural User Interface) = (Think of physical interaction with a GUI)
OUI (Organic User Interface) = (Spend some time researching here)
Important: NUI is NOT a replacement for a GUI but moreover an enhancement allowing a person to physically interact via touch with what they see. Windows 7 will have its GUI but the multi-touch functionality and usability can be thought of as its NUI.