Hi Katie,
The first school that jumps to my mind is ID:
http://www.id.iit.edu/314/
It's more of a PhD in "design thinking" (or insert your favorite
"innovation" buzzword here) than straight up user experience, but as you
noted I don't think you'll find a PhD program in that.
I've know and worked with 2 PhD's from ID (as well as did my undergrad
there, so I'm biased) that I could put you in contact with as well.
FWIW, I've also met a lot of people in the UX profession with PhD in
anthropology as that seems to translate well.
fra