Time Magazine just published a photo essay titled What's Next in Gadgets.

One of the 11 featured gadgets is a cool image of an ASUS notebook with Windows Vista SideShow.
With Windows Vista SideShow notebook users can see if they have new email for instance while the notebook is closed. This works similar to folding phones with two screens. Users can even look up contacts or appointments on the auxiliary screen. Little buttons next to the screen allow you to select and navigate through the available functionality. To enhance the hardware of notebooks just a little bit is a smart marketing move by Microsoft. If you want to be cool you need to buy a new notebook for Windows Vista to have the SideShow display on the outside.
Find more details about Windows SideShow on the Microsoft site. BargainPDA has some more screen-shots.