I have a Dell Vostro v131 that came preinstalled with Ubuntu 11.10. It's not the fastest computer around, but it's decently fast (I develop for Django, App Engine and Plone), reasonably expandable and seemingly well built. Plus, it's cheap enough you can buy one and a Macbook Air for the price of a high-end ugly Windows notebook.

Everything on the V131 works flawlessly.