I've never really busked up here since I'm never here during the season, but boston particularly is a pretty fertile area for street performance. No permit required. I've seen musicians mainly in and around T stations. Living statues are particularly popular here, more than in other east coast cities. The few times I have set up shop for a couple of days here, I did it during the summer in the Boston Commons, near the Park Street stop, as a living statue/dancer. Did pretty well.

As far as Cambridge is concerned, you can be amplified, but there's a $40 permit required. Harvard square, previously a prime spot, has become less and less lucrative due to a combination of gentrification and other changes. I'd say give it a shot- musicians generally have a better time of it there than circle acts do.