Hi - just used my 800 for a family gathering. Had my ISO set at at Auto with a max at 6400. When I used the in camera flash the auto ISO set itself at 6400 even at very closeup shots. Needless to say many shots looked terrible. Had to re-shoot all - shut off the auto ISO, set my ISO to 400 and the new shots look gorgeous. Does the auto ISO always "default" to the highest ISO with in camera flash or did I screwup a setting ?
I had exactly the same experience, twice. At first I couldn't figure out what I was doing wrong. As you found, once I turned of auto ISO, everything was good. Easy solution, but it is a change from the D700.