EMB190 First Officer reports a NMAC with a single engine Cessna during approach to BOS; while outside the Calss B airspace. A TCAS RA commanded a climb alerting the crew.

Narrative

BOS Approach Control vectored us and we joined the 4R localizer at approximately 25 DME. We were then cleared to descend from 10;000 to 6;000 FT. Descending through 8;700 FT a TCAS target suddenly appeared on the MFD TCAS display within approximately 3 miles and 300 FT vertically below. We slowed the descent and then received a climb RA. [We] followed [the] TCAS RA command bars (flight director) until clear of conflict. During the RA maneuver the Captain visually acquired contact with the target aircraft (high-wing Cessna single engine) and estimated our separation as 100 FT vertical and 0 FT lateral. The Captain then relayed to BOS Approach that we had just responded to an RA. The Controller ignored the transmission. The Controller seemed extremely busy at the time. There would certainly have been a collision had we not responded immediately to the RA. This was partly due to us being vectored into the primary class B airport while outside of the class B airspace. Perhaps another Controller 'watching' might have been able to at least give us a traffic alert or issue an advisory stop descent instruction.