Further information for those who are still following this thread: I have now worked out the rest of the mystery.
The problem is not with the iTunes 10.6 installer.
It turns out that I happened to install iTunes 10.6 and iPhone Configuration Utility 3.5 at the same time, with iTunes being installed first. Both of these installers include AppleUSBEthernetHost.kext, and the one included with iTunes Configuration Utility does not work properly.
Therefore, if you have this problem with USB tethering (personal hotspot) not working, the reason is that you have installed iPhone Configuration Utility. The solution is the same as my earlier instructions: move /System/Library/Extensions/AppleUSBEthernetHost.kext to trash, run the iTunes 10.6 installer again, then restart to ensure the old extension isn't still loaded. (After you start up again, you can empty trash.)
The driver has to be deleted manually because both iTunes 10.6 and iPhone Configuration Utility 3.5 install a nominal version 2.3.0 of this driver, with the iPhone Configuration Utility one having a higher internal build number, and the iTunes installer skips installing the driver if it finds the same or a newer version installed.
What we need now is to get Apple to fix the driver included with iPhone Configuration Utility so that this problem doesn't bite too many other people.
maclucassen - can you please update your bug report accordingly?
In addition, there appears to be no problem with USB tethering between iOS 5.1 and older versions of the kext (I succeeded in getting tethering working with kext version 2.2.0, which came with iTunes 10.5.3). My previous test on another computer running iTunes 10.5.3 must have failed for some other reason.