Maya 2009 Mac 10.5.8 I had the above problem. All the correct settings, Keep Faces Together, etc. I created, cut, duped and still when I moved a single vertex, the entire piece would move away from center. Finally I went in and dumped my Maya Preferences file... all better. On one computer I did it with Maya closed and on another where we had the same problem I did it with Maya open. On the open one there was already an object that had been cut down to just front facing faces (think Maya Joan of Arc tutorial) and after eliminating Preferences it started to respond correctly. Yipee.