Massachusetts is one of twelve "two-party consent" states, meaning that persons being taped must consent to their taping.
According to a story in this morning's Boston Globe, some police officers have been using that law to arrest people who are taping them on their cellphones, although convictions, at this point, have been reserved for cases where the taping was done in secret.
Two years ago, Vermont resident Emily Peyton was arrested at a Greenfield, MA anti-war protest for illegal wiretapping, but never prosecuted, primarily because she was very open about her taping of the arrest of a different anti-war protester.
Although video isn't covered by this law, one could imagine that strictly enforcement of this law would impact many businesses, government facilities and law enforcement techniques.
Unless and until this law is changed, the best defense for anyone wanting to document instances of excessive force is to do it as openly as possible.