My heart is broken for those planning to run the Boston Marathon in the heat they are expecting for Monday (see here for ESPN's article called "Heat could affect Boston Marathon"). I would hate to have to face the decision to run in the heat or defer to next year. I would not have the ability to defer if I had already traveled up there and paid for the hotel. I imagine many are in that situation.
For comfort, here is my Chicago Marathon entry where we were also given a Heat Advisory and told to adjust our pace due to the heat. It only made it to the upper 70s, but we had trained in the heat (well, I had in Alabama, so I was pretty accustomed to it). Those running Boston will probably not be as accustomed to the heat, as most of your long runs were in early spring and winter.
I hope you all will make wise decisions about what to do, and please be careful as you are running on Monday. From the perspective of someone who has sat in the medical tent after finishing three marathons and been taken to the hospital before finishing one, NOTHING is more important than your life--even your lifelong dream to run Boston.