Earlier quoted context omitted.
It's unclear to me why Linux running on a retina mac wouldn't have the same resolution problems as Linux running on PC hardware with a similar display.
It absolutely has the same resolution problems. I'm running ubuntu 14.10 on a macbook pro with a retina screen (installed from scratch last week). TweakUI helps make unity look better. Google Chrome's UI looks terrible, although setting the default page scaling to 150% does pretty well for browsing.
ten@vekony:~$ xdpyinfo | grep dimensions
dimensions: 3200x1800 pixels (846x476 millimeters)
I've set the scale to 1.38 for menu and titles bars (in the Screen Display settings) and everything looks great.
Chrome is a nightmare, there are some builds of Chromium that are better with HiDPI support, but Firefox works perfectly with the setting "layout.css.devPixelsPerPx" set to 1.8.
I've also set the text scaling factor to 1.58 (Using Unity Tweak Tool).
There is a good Arch Linux wiki page with some HiDPI tips - https://wiki.archlinux.org/index.php/HiDPI
I'm completely happy with the results and when I use my Wife's MacBook Air I think there is something wrong with my eyes as the difference is really noticeable.