function center(foreground:DisplayObject, background:DisplayObject):void
{
	foreground.x = (background.width / 2) - (foreground.width / 2);
	foreground.y = (background.height / 2) + (foreground.height / 2);
}

http://www.adobe.com/devnet/flash/articles/authoring_for_multiple_screen_sizes.html

Taken from Christian Cantrell's Adobe article called 'Authoring mobile Flash content for multiple screen sizes'.

AS3 Center one DisplayObject relative to another

center

ActionScript 3