The killer loss for me wasn't internet sharing but the version incompatibility - obviously Apple is trying to push you in the direction of getting everyone on the network to install 4.0.1 by making 4.0.1 and 4.0.0 shared libraries incompatible. This strategy would have worked if there was any reason to upgrade to 4.0.1 - network effects being what they are it simply meant that I copied 4.0.0 from one machine to the only machine 4.0.1 had been installed on.