There was a discussion in summer that Ubuntu does not support retina displays - everything on the screen is too small. Even adjusting DPI does not help, since such an adjusting scales only fonts, bit not images.
Is Ubuntu 12.10 already optimized for retina displays. if not is it planned for Ubuntu 13.04?
Agreed, as of 1st Jan 2013 (might be outdated by the time you read this answer, additional info suggestions welcome via comments), there are several good guides (1, 2) addressing a "yay-i-got-it-working" install, but you'll end up with a frankly unusable system where everything is tiny.
So I don't have any answer, but I'm compiling a few facts for anyone with the same question:
Ubuntu 14.04 is quite well adaptable,
Only some java programs don't scale up, but that will come, I guess
Gnome Shell scales pretty good on both low and high resolution screens. Furthermore you can do some fine adjustment changing the fontscale in tweak tool - I set mine to 0,9 on a 1280x800 Thinkpad-Screen - when I was testing Ubuntu 12.04 with Gnome Shell on a MBP about a year ago 1,1 or 1,2 worked fine and looked pretty good.