The newly announced Ultra-Mobile PCs offer a mix of challenges and opportunities, some centring around the native 800 x 480 resolution, and some deriving from the fact that it is at heart a Tablet PC but with the addition of touch. And don't forget the small physical size (and likely to get smaller).
As Alex Yakhnin points out, this is a platform well suited to the skills of Windows Mobile developers.
Since it is running a full version of Windows, some of the software opportunities that exist on the Windows Mobile platform are absent or lessened, since a lot of Windows software will "just work". On the other hand, by comparison with desktop Windows and even the tablet marketplace I think we'll find that some software categories that were not worth attempting owing to the presence of dominant encumbents will suddenly be opened up: if an established application provides a bad user experienced on the UMPC platform, there will be opportunities for nimble-footed newcomers to get in and make some waves.
Or at least I very much hope so :-)
As for my thoughts on the UMPC itself, it's interesting (although no doubt coincidental) that Brighthand is reporting that HP will no longer be offering the hx4700: for many of those of us who lean towards the upper end of the mobile device scale, high-end PDAs lose a lot of their reason for existing with the introduction of the UMPC...we'll finally see an end to most of the necessary compromises that have made those devices in many ways almost useful, but not quite. So the day that was portended at the "Computing on the Beach: Visions of Mobility" panel discussion at PDC 2003 may finally be on the way - my bet is that the high-end PDA will go the way of all flesh (even though it doesn't have any, unless you've been very careless), just as the mini was squeezed between ever more capable PCs and miniaturising mainframes. At the very least I doubt we'll see many more VGA Pocket PCs: while the displays are gorgeous, they suck up a lot of power and introduce technical issues that have impacted software compatibility, and the Windows Mobile implementation of VGA did not give us the kind of resolution improvements many had hoped for. Having said that, there'll no doubt be a swag of VGA Pocket PC announcements next week just to make me look like an idiot...