Then you want getChildByName.

Code:
function regionRollOver(evt:MouseEvent):void
{
	var mc:String = evt.target.name;
	mc = mc.replace("target_", "");
	mc = mc.replace("_mc", "");
	map_mc.getChildByName("target_" + mc + "_mc").alpha = .5;
	map_mc.getChildByName("text_" + mc + "_mc").alpha = .5;
} //regionRollOver()
<Insert standard rant about how naming conventions are awful.>