Charlie, This is the best way I know how to get at it.
import mx.rpc.events.FaultEvent;
import mx.rpc.events.ResultEvent;
import mx.rpc.http.HTTPService;
import com.esri.serialization.json.JSON;
private var httpServ:HTTPService;
httpServ2 = new HTTPService;
httpServ2.url = "http://yourServer/arcgis/rest/services/yourService/mapserver/0?f=json";
httpServ2.addEventListener(ResultEvent.RESULT, xmlResult);
httpServ2.addEventListener(FaultEvent.FAULT, xmlFault);
httpServ2.send();
private function xmlResult(event:ResultEvent):void
{
var Info:Object = JSON.decode(event.result.toString());
var defExpr:String = Info.definitionExpression;
}
private function xmlFault(event:FaultEvent):void
{
var sInfo:String = "Error: ";
sInfo += "Event Target: " + event.target + "\n\n";
sInfo += "Event Type: " + event.type + "\n\n";
sInfo += "Fault Code: " + event.fault.faultCode + "\n\n";
sInfo += "Fault Info: " + event.fault.faultString;
trace(sInfo);
}