It's a great move, but there are some downsides; it's not easy to find the Ubuntu PCs on the Dell site, the easiest route is to follow this link http://www.dell.co.uk/ubuntu or the similar ones for other European countries (list on the Ubuntu site).
More curious, is that if you spec up the Ubuntu equipped Inspiron 6400 to the same spec as that offered with XP or Vista, the price for the Ubuntu laptop is higher (£405.37 vs £398.99).
The basic spec Ubuntu laptop is cheaper (at £329), but adding a comparable processor (single to dual core), memory (from 512MB to 1024MB), and optical drive (DVD/CDRW to DVD+/-RW) makes the Linux laptop more expensive. With pricing like this, smart Linux users will buy the Windows lappy, and install Linux themselves, whilst Dell will drop the Linux option due to lack of demand...
I hope this is just a glitch in Dell's pricing model, component upgrades prior to purchase have always been an area where Dell make their profits. In this case though, it appears that Dell's greed has exceeded the Windows tax. However, my assumption that it is cheaper for Dell to supply a Ubuntu laptop may be wrong, their OEM deal with Microsoft might charge them for every computer shipped.