From http://www.snipplr.com/view/9553/as3-movieclipasabutton-easy-function/

make_button.snippets

// Add any movieclip with (in this example) "my_mc" as the instance name.
 
makeButton(my_mc, my_mc_over, my_mc_out, my_mc_click);
 
function my_mc_over(evt:MouseEvent):void { trace("Rollover"); }
function my_mc_out(evt:MouseEvent):void { trace("Rollout"); }
function my_mc_click(evt:MouseEvent):void { trace("Clicked"); }
 
function makeButton(which_mc:MovieClip, overFunction:Function, outFunction:Function, clickFunction:Function):void {
    which_mc.buttonMode = true; 
    which_mc.useHandCursor = true;
    which_mc.mouseChildren = false;
    which_mc.addEventListener(MouseEvent.MOUSE_OVER, overFunction);
    which_mc.addEventListener(MouseEvent.MOUSE_OUT, outFunction);
    which_mc.addEventListener(MouseEvent.CLICK, clickFunction);
}