Boston, MA Globe, 6/5/12:

The story is here.

Important points here:

It will allow one to ride for free on the rest of the subway system by virtue of the free transfer between the Silver and Red Lines at South Station, provided one originates their trip on the Silver Line (SL1) at the airport. But if I understand correctly, if one plans to continue on the Silver Line (SL5) Washington, which requires leaving the paid system and reboarding another bus at street level, then one pays the fare anyhow on the SL5 bus.

This may have an adverse effect on ridership at the Airport Station on the Blue Line. While the shuttles between the airport and Blue Line station are free, one does have to pay the fare when entering the Blue Line station.

It is aimed at the people who are driving into Logan, not those who are already utilizing public transportation. Saving $2 isn't going to drag people away from the Blue Line, those who are already taking public transportation or who are staying in downtown are still going to use the Blue Line regardless.

Boston, MA Herald, 7/19/12:
Read this.