The Deseret Morning News has a fascinating profile of Paul Cassell, who recently resigned his district court judgeship to return to academia. Just a taste:

  Cassell said he found himself questioning some laws at each turn. "I felt like it was proper judicial role to ask questions, even if we weren't necessarily charged with fixing the problem," he said. But he wanted to do more — he wanted to make a change. Being a federal judge, he couldn't do that.
  "One of the frustrations about being a trial court judge is that you never set broad principles of law; of course, that's reserved for the appellate courts. ... When I was there for 5 1/2 years, I began to think that maybe I would have more effect in moving the law in a way that I think is desirable by doing appellate litigation."

  Thanks to Doug Berman for the link.

  Incidentally, Cassell is one of five federal judges with superlative academic credentials who recently resigned or announced plans to resign either after only a short period of service or when still relatively young. I believe Cassell is now 47, and he resigned in 2007 after 5 and 1/2 years of service. The other four are Mark Filip (41, served for 3 years, now a nominee to become Deputy AG), David Levi (resigned in 2006 at the age of 55 after 17 years on the bench to become Dean of Duke), Michael Luttig (resigned in 2006 at 51 after 15 years on the bench to become GC of Boeing), and Michael Chertoff (resigned in 2005 after 2 years at age 53 to become Secretary of Homeland Security).