I can't vouch for the accuracy of this, but our Comcast cable guy said that we'd be getting fiber in Somerville in ~2012. His impression was that a big part of the issue was that somerville has very little underground conduit and there is a strong preference to bury fiber rather than put it on the telephone poles.

That said, with the exception of one recent issue (not cause by comcast, but by new construction breaking our cable) we've been very happy with Comcast except for their cost.