I have done a hack of the Fate Core system adapting Planescape. I have just finished a complete "alpha version", but couldn't playtest it yet (no time for starting a new group). I'd appreciate any feedback.
It does not follow the Planescape lore to the letter - I assumed that some things, like the Vancian magic, playable races and other stuff are specific to D&D and had no reason to be in the adaptation.
P.S.: Just today I have found another complete adaptation of Planescape for Fate Core, which is frustrating because I searched a lot for something like it before starting and while working. It is available here: http://www.certain-death.com/downloads/