I'll answer my own question: I rented A Cannondale road bike for 2 days from a shop called Back Bay Bicycles on Commonmealth Avenue in Boston. The staff were very helpful and friendly, and included a helmet and lock in the rental. I strongly recommend them if you're in Boston and get the urge to hit the road.
I took a spin around the towns south of Boston and through some national park area, and really enjoyed it. Drivers were courteous (compared to ireland anyway) and the road surfaces were great. The lack of a hard shoulder on the roads is a drawback but overall I had a ball. I wouldn't recommend using any of the bike trails on a road bike: they're too crowded with walkers and joggers and too short to build up a head of team!.