One month ago I bought a Lenovo Ideapad S10. Beautiful, light, small, and fast. I am a very happy owner. The obvious limitations are the size of the screen and keyboard. They are perfect for smurfs, but normal human beings suffer a lot after 3 hours of use.
It came with Windows XP so I was not in a hurry to run Ubuntu. Yesterday, I took a break and installed Ubuntu Netbook Remix, ie, Ubuntu for tiny notebooks. It is perfect and almost solves the screen issue.