How they define underemployment: "Under-Employment Score. The percentage of graduates underusing their skills and credentials. These are graduates who have not successfully started any professional career nine months after graduating law school. We count a graduate as underemployed when he or she is Unemployed – Seeking, pursuing an additional Advanced Degree, in a Non-professional job, or employed in a Short-term or Part-time job."
It should be noted that the Bureau of Labor Statistics showed ~8% unemployment for new advanced degree graduates in October 2011.
It seems to me that, unless you get into a top 5 law school, you have a less than 50% chance of a positive ROI on your tuition money. Hard to say, though.
It would be very interesting to know what would happen if a similar study were done of graduate schools in chemistry.