Are racial and economic diversity substitutes?

7/19/2010: From Ross Douthat in the NYTimes, discussing this book on who gets into elite colleges:

For minority applicants, the lower a family’s socioeconomic position, the more likely the student was to be admitted. For whites, though, it was the reverse. An upper-middle-class white applicant was three times more likely to be admitted than a lower-class white with similar qualifications.