The UMPC as Microsoft defines it: a miniature PC/notebook with a 7" (touchscreen) display with ultra mobility in mind. All the processing power in the world won't matter if your battery doesn't last any longer than an hour.
Here's a good link that I urge everyone to check out if only for a little bit:
You'll see every UMPC in a popularity list. The most popular form factor seems to be the clamshell type with the maybe-full-but-mostly-crippled keyboards on a lot of the models.
For many of them, they resemble miniature slate-tablet PCs. It wouldn't matter if they didn't have keyboards then, because there's a text input panel on-screen and full handwriting recognition in Vista and XP Tablet Edition.
The biggest one of 'em all is the HTC Shift. The sleekest design, a lot of power and features, and a smartphone asthetic about it make it one of the most popular around, even though it probably won't be released 'till the end of the year. It's got dual processors in the true sense of the word. It runs both Windows Mobile 6 and Vista Business on two processors simultaneously. With a push of a button, the screen will fade and switch in between the two. So while you're still booting up Vista, you can check the weather or browse the net or check your contacts and such on the other processor. Realllllly cool (nerd) stuff.