In Mac OS X default package[1] shabang in /usr/local/bin/hg has been set to `#/usr/bin/python` invoking, then, default system python installation. If a user, quite common scenario, installed another, updated, python version, extensions installed by easy_install bound to custom python isntallation fail to be loaded. To fix it it's enough to change shabang to `#/usr/local/env python` [1] http://mercurial.berkwood.com/
We actually construct the shebang line as part of the install process. Your problem was that you installed dulwich (required by hg-git) in your upgraded Python, which is not the Python used by the OS X packages provided by berkwood. Using /usr/bin/env python will actually make things worse in the presence of stuff like virtualenv.
I am sorry. I discussed the issue in IRC and they explain me the problem when switching to VirtualEnvs. The solution is to install extensions using /usr/bin/easy_install so that Mercurial environment and development one are separated environments. Thanks for your patience.
--- Bug imported by bugzilla@serpentine.com 2012-05-12 09:31 EDT --- This bug was previously known as _bug_ 3437 at http://mercurial.selenic.com/bts/issue3437