I've had a hunch for some time an initiative like this would emerge, despite classic "first-mover" hurdles. Reactions to Yale Law School's Ph.D. in Law program were both swift and varied. While Brian Leiter was quick to describe YLS's move as "the worst idea in the history of legal education," other commentators offer a more measured, "wait-and-see" take (e.g., here, here, and here).
If nothing else, Yale's effort represents another response (and a not implausible alternative to pursuing a J.D/Ph.D. in a traditional non-law discipline) to palpable changes to the law faculty entry-level hiring market. As YLS Dean Robert Post correctly notes, "You have to come in now with a portfolio of writing. People [law school hiring committees] require you to show your abilities as a scholar by what you've written."