Of course, thank Flash F1 help for this:
"Converts the pt object from the movie clip's (local) coordinates to the Stage (global) coordinates.
The MovieClip.localToGlobal() method allows you to convert any given x and y coordinates from values that are relative to the top-left corner of a specific movie clip to values that are relative to the top-left corner of the Stage."
var myPoint:Object = {x:this._x, y:this._y};
this.localToGlobal(myPoint);
var stagex:Number = myPoint.x;
var stagey:Number = myPoint.y;
trace(stagex);
Ok, so define your point as an object, with the properties x and y.
Then the function changes your point's x and y properties to be relative to the _root stage, and then you've got your new x and y!