Yeah, you can definitely get Linux running on pretty much whatever. But the point would be for Ubuntu to provide something that requires absolutely no fiddling, and where everything is nicely tuned and tested, and hey, maybe with some really kick-ass hardware at that.
I'm in the market for such a machine, and it's driving me absolutely batty.
* The Dell XPS 13 is the front runner, but I would prefer a larger screen. I stare at it all day, so that's more important than having something super portable. Also, compared to my current 1900x1200 screen, I'd be taking a pixel hit going to 1900x1080. Because some morons decided that watching movies was more important than doing actual work with computers. Also, the Wired review does mention Wi-Fi problems with the machine, which are not Linux specific, but probably due to the construction. http://www.wired.com/reviews/2013/07/dell-xps-linux/
* The Chromebook Pixel. Now this is a screen! I love it! The whole package looks pretty nice, although it's maybe a tad underpowered in terms of CPU and memory - I'd spend a bit more to get something I'm going to be comfortable with for the next few years. The killer though: 32 gigs of storage! NOT OK. Yeah, it comes with 'Google Drive' cloud stuff, but that doesn't cut it for me. I don't use a lot of disk - my current machine only has 100 gigs used up, so I'd probably be able to get by with a 128 gig SSD, but 32?! Sorry. Also, I'm a bit worried about support for the machine: Dell's has, for me, been pretty good in the past. I'm not sure I trust Google as much.
* There's a new Samsung that's supposed to come out with a high res screen soon: http://www.samsung.com/global/ativ/ativbook9plus.html - it looks pretty good, but once again, 13 inches... not sure I'd be happy with that, and I'm not sure it'll be out in time for me to get it during my trip to the US:-/
Sure, I can get an external screen, but that means I have a completely different screen environment depending if I'm at home, at work, or somewhere else.