LABYRINTH: Medieval Studies Organizations
This isn't up to date, but it's a good place to start. Not all of the universities listed offer a formal major. Harvard, for instance, only has a minor ("secondary topic"). Ditto Cornell. But most schools also have design-your-own-major programs which can easily be used to adapt a minor into a major if there are adequate appropriate courses offered -- which will be true in any decent-sized university.
EDIT: I think you have to look critically at some of this stuff. There seem to be very few "majors" out there, and Rice doesn't require any language as part of its major, which is kind of an instant credibility-destroyer.