Heard of Dell XPS developer edition?
Absolutely I have but a) people I know with them have still had silly issues b) they came at it from the wrong end, Dell decided to adopt Ubuntu and did some minimal work with Canonical.
IMO Canonical needs to commit to a partnership with a vendor to produce a machine they can fully focus and support on, that way you get a product you know will just work and can be polished to a high degree.
I'm running Xubuntu on an old(ish) Vostro 3750 and with the exception of it's somewhat mediocre touchpad (Alps sucks) literally everything works exactly as you'd expect but that wasn't the case when this was a new machine (certain bits where flaky but got fixed over the following year), if they could provide the experience I have now on a 3 year old machine on a brand new machine with top of the line hardware then they'd be onto a bit of a winner, it would certainly top any list I'd look at for a while since my primary requirement for any hardware is does this work properly on Linux already.