Everything I’ve read says that the USB tethering facility of Android phones doesn’t work with Macs without installing additional software and possibly going through various moderately elaborate procedures. Imagine my surprise when I accidentally turned it on and it worked perfectly.
It appears that this is an unadvertised feature of Missing Sync for Android, which does use the phone’s tethering function as part of their recently added sync-over-USB feature. Specific evidence: (1) The phone does not appear as a network interface on another Mac with the same OS version. (2) When I unplug the phone or disable tethering, the network interface changes its name momentarily from "USB Ethernet (en3)" to "MissingSync" before disappearing.
I’m using Android 2.2.1 (FRG83D) on a Nexus One, Mac OS X 10.5.6, and Missing Sync 1.5.1 (phone)/1.5.0 (desktop).
I reiterate that this is not documented anywhere I have found, and especially not on Mark/Space, Inc.’s web site.