Ok, since this is also a blog for all future exchange students, here are some thoughts and tipps on how to find a room in Boston:
first of all, you should probably check the alumni reports for tipps or adresses of former hosts, because not only did those hosts then probably have a good experience with exchange students, the students can also tell you whether or not everything worked out and what you might have to take into consideration when chosing this particular host. I met the guy who stayed with my host last winter by chance, actually. I already had the room after I found out that another student from Frankfurt had stayed in it a year before :) It's a small world after all.... anyways, we met at our universitie's USA exchange party and he gave me a few of his impressions on my host and my surroundings, which I really apprechiated.
I found my room via the MIT European Club website. They have a "marketplace" where students and Bostonians offer their home to exchange Students. This is very practical, since it is designed to be especially for students staying for a short period of time (apparently other appartments and rooms are only availably for a year's lease...) So, if you don't already have a personal contact, check out http://euroclub.mit.edu/marketplace and see if you can find something there!