private const RADIUS_OF_EARTH_IN_MILES:int = 3963;
private const RADIUS_OF_EARTH_IN_FEET:int =20925525;
private const RADIUS_OF_EARTH_IN_KM:int =6378;
private const RADIUS_OF_EARTH_IN_M:int =6378000;

private function distanceBetweenCoordinates(lat1:Number,lon1:Number,
                                            lat2:Number,lon2:Number,
                                            units:String="miles"):Number{

	var R:int = RADIUS_OF_EARTH_IN_MILES;
	if (units == "km"){
		R = RADIUS_OF_EARTH_IN_KM;
	}
	if (units == "meters"){
		R = RADIUS_OF_EARTH_IN_M;
	}
	if (units =="feet"){
		R= RADIUS_OF_EARTH_IN_FEET;
	}

	var dLat:Number = (lat2-lat1) * Math.PI/180;
	var dLon:Number = (lon2-lon1) * Math.PI/180;

	var lat1inRadians:Number = lat1 * Math.PI/180;
	var lat2inRadians:Number = lat2 * Math.PI/180;

	var a:Number = Math.sin(dLat/2) * Math.sin(dLat/2) + 
                       Math.sin(dLon/2) * Math.sin(dLon/2) * 
                       Math.cos(lat1inRadians) * Math.cos(lat2inRadians);
        var c:Number = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 
        var d:Number = R * c;

	return d;
}

http://www.terrenceryan.com/blog/post.cfm/getting-the-distance-between-two-points-using-actionscript

Full credit for this goes to Terrence Ryan who converted the JavaScript version to ActionScript.

AS3 Getting the Distance Between Two Geographical Points

ActionScript 3