This piece of script shows how to use dynamic text as a mask. Create mc1, which can be a picture or simple a coloured background and link it. Create mc2, which is a dynamic textfield converted to a movieclip and link it. Then put this script for example in a keyframe.

attachMovie("colormc", "mc1", _root.getNextHighestDepth(), {_x:25, _y:25});
attachMovie("textclip", "mc2", _root.getNextHighestDepth(), {_x:25, _y:25});
var mc1:MovieClip;
var mc2:MovieClip;
var i:Number = 0;
mc1.setMask(mc2);
mc2.onEnterFrame = function() {
	i++;
	this.myText.text = i;
};