Honestly, I don't get the allure of ultramobile PCs. Neither small enough to be pocketable nor large enough to work on comfortably, most seem to me to be expensive toys. Although some serve needs in specific industries--medical, retail, automotive, and military applications--historically, these platypus PCs have had bizarre (but creative) keyboards and have squeezed the standard Windows interface onto screens really not designed for the desktop experience.
But with Intel's new low-power and low-cost Atom CPU, the prices for these machines are coming down to almost reasonable levels. And many vendors, realizing that no standard mouse-and-keyboard-based UI is suitable for this form factor, are releasing their products with new, iPhone-like interfaces that are a better for their touch screen displays.
All the new ultramobiles come with wide-area networking technologies (either HSDPA, EVDO, or WiMax), which makes them potentially very interesting Web application platforms. To my knowledge, though, there are no HTML standards nor generally accepted guidelines for writing a Web app for a touch-screen interface, except for what Apple is doing. This will make using these products as Web clients frustrating at first.
Several of these new devices were on display at the Intel Developer Forum. Many of the products are currently for sale in Asian countries, but some are coming to the U.S. later this year or early next. Click through to the embedded gallery for a tour.