Dell is about to unveil two consumer PCs in Europe - the Inspiron 6400n (£329 including VAT and delivery) and the Inspiron 530n (££399 ditto) - with the Ubuntu 7.04 Linux operating system factory installed.
These prices don't include any software support, but you can sign up with Canonical for $250 or (for 24x7 support) $900 a year.
Dell says to see http://www.dell.co.uk/ubuntu but it doesn't work at the moment. You could try here