Going to med school
The decision was well-cemented in my mind by the time I woke up this morning. Lots of people helped me come to it; thanks to all. Basically, while the location isn't as interesting as Harvard Square, I find that I can't pass up the opportunity to apply my skills to hard science, especially in a field as interesting to me as genome-related research. (And the location isn't horrible, either; at least it's near the T.)
It was hard writing the genuinely regretful turn-down letter to the ICG. I hope they find someone else who is as interesting as me, ho ho. (I don't doubt that they will, actually.)