Twelve-inch screens aren't a lower limit, by any means. Ultra-mobile PCs are on everybody's radar, even if everybody doesn't agree what a UMPC is.

What they've been so far has been frankly experimental. The early examples have included the Samsung Q1 with a 7-inch screen, the Fujitsu U810 with a 5.6-inch screen, and the Palm Foleo with a 10-inch screen and a touch-typist's keyboard. And there will be a lot more experimentation to come next year.

There are a couple of ways to look at UMPCs. One is that they're a continuation of the downsizing of notebooks. Gartner's Fiering sees them as devices for "walking workers" -- nurses, lab techs, even delivery-truck drivers -- who can take advantage of their touchscreen support to provide a migration path from larger tablet machines.

Another viewpoint is to see UMPCs as an upsizing of extremely small devices (smartphones and PDAs and Pocket PCs) with tiny screens and no keyboards. Web browsing is becoming the primary function of any device with a display screen, and larger UMPC devices, with screens big enough to read, and keyboards big enough to use, look like winners. This matches Paul Moore's vision of an "unconsciously portable" device -- something so small and light you'll carry it everywhere, yet big enough that you can see the screen, type on the keyboard, use your familiar productivity applications, and connect to your company network.

The Holy Grail of UMPC design -- small enough but big enough, portable but usable - may not be found in 2008. The limitation of small batteries, tiny hard drives, reduced-speed processors, and screens that range from small to smaller, means that UMPCs won't find mass-market acceptance next year. Fiering predicts that we're a year away from better prices and two years away from hardware designs and features that will make UMPCs an established category.

VIA's Richard Brown splits UMPCs into two groups on either side of what he calls the "6-inch divide." Above that screen size he sees devices with screens about 7 inches on the diagonal, and standard keyboards. It's here, he says, that his company's low-power processors are currently finding enthusiastic customers.

Below the 6-inch divide are devices with screens 5 inches and smaller that will have thumb keyboards and stylus-driven interfaces. "The OQO is an example," he says. "These little machines are strong on connectivity -- some U.S. models have EV-DO -- and data-handling and browsing are very good. Power consumption needs to be improved."

"You'll see five-inch screens next year," says Brown, "then smaller. It's a two- to three-year trend, it won't happen overnight." But the end result will be a convergence of these ultra-mobile devices with smartphones like the iPhone and the recently announced Open Moko Linux platform.