Are you considering an ultra-mobile PC this holiday season? If so, then I highly recommend you check out the 11,000 word, 28-page Ultra Mobile Computing Buyer's Guide put together by the biggest fan of UMPCs I have ever seen, Steve "Chippy" Paine. He initially posted this in a 4-part article and has now assembled it all into a single PDF document that you can download and use as an essential reference tool.
I was reading through this outstanding guide this morning and am very impressed by the amount and quality of information Steve provides. You can read about the history of ultra mobile computing, different features, the different processors, different form factors, and more. There is a link to his incredible UMPC list that shows just about every ultra portable computer you can think of and then provides more details and specs when you click on the image on the page.