Check out the ultimate ultra mobile PC buyer's guide

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.