[7thsky™]

import mx.core.ext.UIObjectExtensions;
import mx.controls.ComboBox;
UIObjectExtensions.Extensions ();

createClassObject (ComboBox, 'a', 1);

var o:Array = new Array ();
for (var i:Number = 0; i < 10; i++) {
	o.push ( {label: 'test' + i, data: i} )
}

a.dataProvider = o;

this.i = 0;
this.onEnterFrame = function () {
	if (++this.i > 10) {
		delete this.onEnterFrame;
		
		this.a.selectedIndex = 5;
	} // end if
}

а ты проверь?