We should have a reasonable expectation of privacy when we are in a private place. Say our home, in our car with the windows closed, etc.
There is no way, though, that police should ever have any expectation of privacy while performing duties on behalf of their public employers. It is pretty ludicrous that in one jurisdiction police have charged someone with wiretapping when he recorded them while being investigated for drunk driving:

Police say they were patrolling the downtown area at 2:54 a.m. when they discovered Christopher A. Power of 52 Chestnut St. sitting in the driver’s seat of a vehicle with its motor running at the Rochester Common.
After speaking with Power, police began investigating him for driving while intoxicated and arrested him. During the arrest an audio recording device was discovered.
“During a search after the arrest an audio recorder was discovered on the driver’s seat cushion,” Capt. Paul Callaghan said. “The officer noticed that the recorder was recording.”
Power was charged with driving while intoxicated and wiretapping, which is a Class B felony.

Perhaps they were doing their investigation via cell phone. Or perhaps they live in an alternate universe where wiretapping means something very different.
That aside, police must always expect to be audio- or videotaped when they are on duty.

Via Balko.