Agreed, as of 1st Jan 2013 (might be outdated by the time you read this answer, additional info suggestions welcome via comments), there are several good guides (1, 2) addressing a "yay-i-got-it-working"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:
- Window Manager / Desktop Environment:
- Reports (1, 2) indicate that GNOME Shell (and KDE) currently manage High-DPI better than Unity 12.10. You'll suffer from tiny icons in applications, but Shell itself will correctly scale.
- Discussions happened on summer 2012 in this gnome-devel-list thread, but to sum up: nothing happens because developers have no test hardware; if you want progress done, buy them a MBP Retina.
- A similar thread was brought up on the unity-design mailing list, but did not go much further.
- GTK: Bugzilla has Bug 546711, which hasn't seen lots of activity recently. Summary: a patch was proposed years ago but wasn't satisfying. Properly fixing the bug requires more work, probably down to the X level.