UMPCs are supposed to be small, lightweight "companion" devices that let their owners have many of the features of a PC with a smaller form factor and smaller price tag.
Having seen and used several UMPCs, they are neither small enough to fit into a pocket or robust enough to do the majority of features on a tablet PC. They can be used for getting contacts, surfing the internet, checking and sending email and listening to music or watching videos. The screens are too small for reading most documents and any regular work.
I don't own one (yet), but having played with the iPhone, it does all the above pretty well at a lower cost and smaller size. As a bonus, you can make phone calls!
While the premise for the origami or UMPC is interesting, I think the Microsoft and Intel folks missed the mark. Apple's iPhone is nearly everything I'd expect and ultra mobile PC to be and I think they've set the bar for the convergence of the phone, computer, and multimedia electronics.