According to Apple's Darwin site, OS X 10.1 should have Darwin version 1.4.1. According to my copy of 5g64, it says it has only version 1.4, and the dev tools have errors that I can't believe Apple would allow to ship out the door. So I think it's quite possible that there are newer builds.
Mac OS X 10.1 ships with Darwin kernel 1.4. The next standalone Darwin release, which is almost exactly the same, will be 1.4.1. The .1 accounts for the fact that standalone Darwin has some code disabled because it cannot be open sourced.
Software updates should bump the third version as well, and Darwin will then leapfrog again.
While it wouldn't supprized me that 5g68 is out there in Apple some place, I seriously doubt that it is or was suppose to be 10.1. It may have been an internal Apple build for them to try some things out but since they have hit milestone with 5g64, the rest of the builds in that series will be 10.1.x upgrades.
Apple email's to the developers state that 10.1 is 5g64 and the reports I have been hearing from people who have the cd's that were distributed at the show are stating the same thing.