i have a lenovo ideapad s10e and im very happy with it.
heres a pic: http://www.adammaguire.com/blog/wp-content/uploads/2009/02/ideapad.jpg
i can totally recommend it^^
its not as stylish as other netbooks but its sturdy and reliable.
im running windows 7 on it so ubunto shouldnt be an issue.
the hardware is the same as in other netbooks so you shouldnt have any driver issues, too.
Any of the NetBooks can run Ubuntu (or similar Linux's) quite well. Most of the current ones are based on the n270 Atom CPU, older ones used n230s or a special low-power Celeron. I would suggest trying to get your hands on one with the n270 - the extra bit of power is handy, but do bear in mind that they are a small fraction as powerful as normal CPUs.
Going for the refurb's is a good idea if you can get them from a reputable place. Bigger SSDs are good, and there's nothing wrong with a normal hard drive either. If possible, try to get one with 2g of RAM.