Mentioning this here because it’s hard to find with Google unless you speak Japanese.
http://homepage.mac.com/mkino2/backnumber/2005_11.html has a good example of drag and drop in a NSTableView powered by CoreData.
http://homepage.mac.com/mkino2/backnumber/image/2005_11/DNDCoreData.pdf - in English
http://homepage.mac.com/mkino2/backnumber/image/2005_11/CoreDataDND.zip - drag and drop example code with e-mail entities
http://homepage.mac.com/mkino2/backnumber/image/2005_11/CoreDataDND2.zip - example code, if you need different ordering in each of several categories (adding an “ordered e-mail” wrapper entity)
(Sorry for the lousy formatting. Just wanted to make this googleable; no time to learn the formatting right now.)