Interdisciplinary PhDs? Is there such a thing as an interdisciplinary PhD, where the student chooses the fields, courses, etc.? If so, which U.S. universities offer such a thing?
For a while I was a graduate student in the math department of Portland State University. At Portland State, the Ph.D. program in math requires students to have an "allied area" (field of study other than mathematics). The student's thesis must be related to this allied area, the student must take about 25% percent of their courses in the allied area, and the student must pass a qualifying exam in the allied area. Thus the whole program is designed to be interdisciplinary from the get-go. 

(Side note: The actual name of degree is "Ph.D. in Mathematical Sciences", and it definitely differs from the traditional math Ph.D.)