This is a quick guide for buying a mid level laptop (based on recommendations given to one of my friends recently).
For ultra portable perhaps asus eee pc is a clear leader (for now) http://kakaku.com/item/00200916376/
A very nice site for comparing the prices is http://kakaku.com/sku/pricemenu/winn.htm
Note: sometimes you can get quite a bit better deal by clicking on one of the advertisements on the page.