HTTPService/HTTPMultiService have a cool concept of a Serialization Filter. This mechanism let’s you configure and manipulate various aspects of the HTTP call like the request, response, etc..
A default implementation of SerializationFilter is provided in mx.rpc.http.SerializationFilter

Usage -
You can create a custom SerializationFilter by extending from the default implementation and set it on the HTTPService instance or the Operation instance.

A sample implementation can be as simple as processing the response from the service.
For example, if your service returns a comma (,) separated list of values, you can convert it to an Array here.

package
{
import mx.rpc.http.AbstractOperation;
import mx.rpc.http.SerializationFilter;

public class CustomSerializationFilter extends SerializationFilter
{
	override public function deserializeResult
		(operation:AbstractOperation, result:Object):Object
	{
		var arr:Array = [];
		if (result is String)
		{
			arr = String(result).split(',');
		}
		return arr;
	}
}
}

And no, this is not all, there are other methods also which exposes more functionality.

For example, you can use the serializeURL() method to process/modify the url which this service/operation is about to invoke.

Livedocs here