They both are interested in Justice Thomas Ruffin's 1830 opinion in State v. Mann! Sally writes about it and Obama taught it in his seminar on current issues in racism and the law at the University of Chicago back in 1994. How do we know this? The syllabus for the course is up on the New York Times' website. Pretty interesting set of readings!
I have some more thoughts on the syllabus--and why it's not getting more attention--over at the faculty lounge.