In Chicago, a lot of data has been released on the City's Data Portal, but I have no idea how much is left to be opened.
The strategy I've noticed governments seem to follow is to start with a department head that is convinced open data is good and work their way from department to department, liberating one dataset at a time. As more departments get on the open data bandwagon, the ones on the fence see the benefits and fall in line.
Are there other strategies? Has any city or municipality successfully opened all their available data? If so, when did they know they were done?