My guess is that from Dells perspective, this is partly an experiment and partly a way to please the masses over on IdeaStorm. Dells decision not to allow Linux to openly compete with Windows is an indication that the company doesnt want to sour any relationships with Microsoft (and lets face it, if Microsoft were to add an extra $5 or $10 to the price of a Vista Home Premium license, it would have a severe impact on Dells profitability). If, over the next five years Dell discovers that there is in fact a market for Linux, they can take it from there, if not, they can shelve the idea and say that they tried.
This is also a safe route for Ubuntu and Canonical. By selling Dell rigs to existing people already in the Linux loop, Canonical doesnt risk having to face angry users and explain why Ubuntu wont run their favorite Windows applications.