Simply rebranding basic hardware isn't going to compete with the likes of Apple.

Cheap and cheerful (read: shoddy) hardware would quickly damage the Ubuntu brand, and the good hardware is locked up tight by the big brands. Canonical would need to put in some serious volume or R&D of their own to get good hardware that would lead to a viable Ubuntu hardware brand.

Ubuntu users, at least at the moment, are quite picky about the specs of their PCs.