The Boston Globe reports that police n Massachusetts are using the state's wiretapping laws to arrest people who record cops on the job. Massachusetts is one of 12 two party consent states, which cops are interpreting to mean you can't record them without their permission.
The state's supreme court upheld such a conviction in 2001, finding that "Secret tape recording by private individuals has been unequivocally banned, and, unless and until the Legislature changes the statute, what was done here cannot be done lawfully." I'd think you could make a strong case that a public employee entrusted with the power to forcibly detain and kill falls outside the scope of a "private individual."
According to the Globe, subsequent cases have turned on whether the recording was done secretly (in which case convictions are usually upheld), or openly (in which case the charges are usually dropped).
Boston police are claiming that recording them while on duty violates their privacy rights and may interfere with their ability to make arrests.