The problem seems to be located within one of the programs called by:
which were updated by the 10.7.4:
It is a real shame when you think that this daemon (airportd) is still only able
to provide WEP, 10 years after the public knowledge than this WEP is just:
If someone suggest you to play with your preferences
or to stop and start InternetSharing, be warned, this won't hold for more
than 2 minutes.
If you have a backup of 10.7.3: just roll back.
If someone suggest you to reinstall your system from 10.7 and make all
the update till the 10.7.3, suggest hime to correctly evaluate the waste of time