http://geekhack.org/index.php?topic=41422.0
Another interesting project to follow is the Nexus keyboard : http://geekhack.org/index.php?topic=44940.0
To me one of the rare person who really "got it" is "Jesse" (he's both here, on GeekHack and on Deskthority) with its "butterfly" keyboard.
That's one of the only split (and ergonomic) keyboard that takes into account the fact that the thumb's opposition-apposition makes it unlike any other finger.
Look at his "butterfy" keyboard here and how smartly the keys you're to hit with the thumbs are positionned:
http://blog.fsck.com/2013/12/better-and-better-keyboards.htm...
I agree that the ergodox is not perfect and the thumb clusters could be improved, I'm just not sold that the butterfly does that.
Here's an old post with my initial impressions of the ergodox [0]. I need to add an update, but I have since purchased a second unit for home and have been typing with the "Norman" layout since day 2.
(BTW, massdrop currently has the keyboard up for sale, but you need to act fast as there is only a day or so left)
0 - http://hairysun.com/blog/2013/04/02/oh-ergonomic-keyboard/
Separately, I've very gradually been working on a personal project starting with slightly modified Kinesis keywells for the fingers. (I've been using Kinesis keyboard for 15 years and am mostly satisfied with the finger portion.) I use small SPI / I²C adapters on the keywells for flexible separation and positioning². I have still not found a thumb arrangement I'm happy with, though.
The AcidFire Nexus design linked elsewhere in this thread attempts to address this by dropping the thumb clusters to a lower level, which looks like a really great idea. The only other design I've seen which addresses this problem is Oobly's: http://geekhack.org/index.php?topic=49721.0