If someone sold hardware that came with some guarantee that a major linux distro had 100% support of all the hardware I would definitely consider buying it. I use ThinkPads because their support is pretty good (and I like the physical device) but there is always some trade off when it comes to hardware and the drivers.

Probably not (many), but I'd wager a thing like this is largely aimed at current non-Linux users anyway.

Alternately, Linux by default could be seen as a sign that hardware is reasonably well supported, so rolling one's own may be less effort than otherwise.