is there a way to reset the undo history?

FCKUndo.resetUndoState = function()
{
   //simple reset of the undo history
   this.SavedData = new Array() ;
   this.CurrentIndex = -1 ;
   this.TypesCount = 0 ;
   this.Changed = false ;
   this.MaxTypes = 25 ;
   this.Typing = false ;
   this.SaveLocked = false ;
   FCK.Events.FireEvent( "OnSelectionChange" ) ; // this updates the undo and redo toolbar status
}

To make this a really useful solution .. I had to modify fck.js to add a new function ..

   ResetUndoState : function()
   {
      FCKUndo.resetUndoState() ;
   },

You can then call the reset using something like oEditor.ResetUndoState();
I also simplified the function in FCKundo.js

FCKUndo.resetUndoState = function()
{
   //simple reset of the undo history
   this.SavedData = new Array() ;
   this.CurrentIndex = -1 ;
   this.TypesCount = 0 ;
   this.Changed = false ;
   this.Typing = false ;
   this.SaveLocked = false ;
}

You need to call the FCK.Events.FireEvent( "OnSelectionChange" ) event if you use ResetUndoState inside a plugin. It's not needed if you call it through your instance reference (EG oEditor.ResetUndoState())

Anyways .. I post this in case anyone ever needs it in the future